[Mono.Android] fix a set of the "easiest" trimmer warnings

src/Mono.Android/Android.Graphics/Color.cs

@@ -395,11 +395,18 @@ public static void RGBToHSV (int red, int green, int blue, float[] hsv)
 
 	public class ColorValueMarshaler : JniValueMarshaler<Color>
 	{
+		const DynamicallyAccessedMemberTypes ConstructorsAndInterfaces = DynamicallyAccessedMemberTypes.PublicConstructors | DynamicallyAccessedMemberTypes.NonPublicConstructors | DynamicallyAccessedMemberTypes.Interfaces;
+		const string ExpressionRequiresUnreferencedCode = "System.Linq.Expression usage may trim away required code.";
+
 		public override Type MarshalType {
 			get { return typeof (int); }
 		}
 
-		public override Color CreateGenericValue (ref JniObjectReference reference, JniObjectReferenceOptions options, Type targetType)
+		public override Color CreateGenericValue (
+				ref JniObjectReference reference,
+				JniObjectReferenceOptions options,
+				[DynamicallyAccessedMembers (ConstructorsAndInterfaces)]
+				Type targetType)
 		{
 			throw new NotImplementedException ();
 		}
@@ -414,6 +421,7 @@ public override void DestroyGenericArgumentState (Color value, ref JniValueMarsh
 			throw new NotImplementedException ();
 		}
 
+		[RequiresUnreferencedCode (ExpressionRequiresUnreferencedCode)]
 		public override Expression CreateParameterToManagedExpression (JniValueMarshalerContext context, ParameterExpression sourceValue, ParameterAttributes synchronize, Type targetType)
 		{
 			var c = typeof (Color).GetConstructor (new[]{typeof (int)})!;
@@ -424,6 +432,7 @@ public override Expression CreateParameterToManagedExpression (JniValueMarshaler
 			return v;
 		}
 
+		[RequiresUnreferencedCode (ExpressionRequiresUnreferencedCode)]
 		public override Expression CreateParameterFromManagedExpression (JniValueMarshalerContext context, ParameterExpression sourceValue, ParameterAttributes synchronize)
 		{
 			var r = Expression.Variable (MarshalType, sourceValue.Name + "_p");
@@ -433,6 +442,7 @@ public override Expression CreateParameterFromManagedExpression (JniValueMarshal
 			return r;
 		}
 
+		[RequiresUnreferencedCode (ExpressionRequiresUnreferencedCode)]
 		public override Expression CreateReturnValueFromManagedExpression (JniValueMarshalerContext context, ParameterExpression sourceValue)
 		{
 			return CreateParameterFromManagedExpression (context, sourceValue, 0);

src/Mono.Android/Android.Runtime/IJavaObjectValueMarshaler.cs

@@ -10,9 +10,16 @@ namespace Android.Runtime
 {
 	sealed class IJavaObjectValueMarshaler : JniValueMarshaler<IJavaObject> {
 
+		const DynamicallyAccessedMemberTypes ConstructorsAndInterfaces = DynamicallyAccessedMemberTypes.PublicConstructors | DynamicallyAccessedMemberTypes.NonPublicConstructors | DynamicallyAccessedMemberTypes.Interfaces;
+		const string ExpressionRequiresUnreferencedCode = "System.Linq.Expression usage may trim away required code.";
+
 		internal    static  IJavaObjectValueMarshaler              Instance    = new IJavaObjectValueMarshaler ();
 
-		public override IJavaObject CreateGenericValue (ref JniObjectReference reference, JniObjectReferenceOptions options, Type? targetType)
+		public override IJavaObject CreateGenericValue (
+				ref JniObjectReference reference,
+				JniObjectReferenceOptions options,
+				[DynamicallyAccessedMembers (ConstructorsAndInterfaces)]
+				Type? targetType)
 		{
 			throw new NotImplementedException ();
 		}
@@ -27,6 +34,7 @@ public override void DestroyGenericArgumentState ([AllowNull]IJavaObject value,
 			throw new NotImplementedException ();
 		}
 
+		[RequiresUnreferencedCode (ExpressionRequiresUnreferencedCode)]
 		public override Expression CreateReturnValueFromManagedExpression (JniValueMarshalerContext context, ParameterExpression sourceValue)
 		{
 			return Expression.Call (
@@ -36,6 +44,7 @@ public override Expression CreateReturnValueFromManagedExpression (JniValueMarsh
 				sourceValue);
 		}
 
+		[RequiresUnreferencedCode (ExpressionRequiresUnreferencedCode)]
 		public override Expression CreateParameterToManagedExpression (JniValueMarshalerContext context, ParameterExpression sourceValue, ParameterAttributes synchronize, Type? targetType)
 		{
 			var r   = Expression.Variable (targetType, sourceValue.Name + "_val");

src/Mono.Android/System.Drawing/PointConverter.cs

@@ -30,6 +30,7 @@
 
 using System.Collections;
 using System.ComponentModel;
+using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
 using System.ComponentModel.Design.Serialization;
 using System.Runtime.CompilerServices;
@@ -133,6 +134,7 @@ public override bool GetCreateInstanceSupported (ITypeDescriptorContext context)
 			return true;
 		}
 
+		[RequiresUnreferencedCode ("The Type of value cannot be statically discovered.")]
 		public override PropertyDescriptorCollection? GetProperties (
 							ITypeDescriptorContext context,
 							object value, Attribute[] attributes)

src/Mono.Android/System.Drawing/RectangleConverter.cs

@@ -31,6 +31,7 @@
 
 using System.ComponentModel;
 using System.Collections;
+using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
 using System.Text;
 using System.ComponentModel.Design.Serialization;
@@ -147,6 +148,7 @@ public override bool GetCreateInstanceSupported (ITypeDescriptorContext context)
 			return true;
 		}
 
+		[RequiresUnreferencedCode ("The Type of value cannot be statically discovered.")]
 		public override PropertyDescriptorCollection? GetProperties (
 							ITypeDescriptorContext context,
 							object value, Attribute[] attributes)

src/Mono.Android/System.Drawing/SizeConverter.cs

@@ -31,6 +31,7 @@
 
 using System.Collections;
 using System.ComponentModel;
+using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
 using System.ComponentModel.Design.Serialization;
 using System.Reflection;
@@ -135,6 +136,7 @@ public override bool GetCreateInstanceSupported (ITypeDescriptorContext context)
 			return true;
 		}
 
+		[RequiresUnreferencedCode ("The Type of value cannot be statically discovered.")]
 		public override PropertyDescriptorCollection? GetProperties (
 							ITypeDescriptorContext context,
 							object value, Attribute[] attributes)

src/Mono.Android/System.Drawing/SizeFConverter.cs

@@ -32,6 +32,7 @@
 using System;
 using System.Collections;
 using System.ComponentModel;
+using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
 using System.ComponentModel.Design.Serialization;
 using System.Reflection;
@@ -118,6 +119,7 @@ public override bool GetCreateInstanceSupported (ITypeDescriptorContext context)
 			return true;
 		}
 
+		[RequiresUnreferencedCode ("The Type of value cannot be statically discovered.")]
 		public override PropertyDescriptorCollection? GetProperties (ITypeDescriptorContext context, object value, Attribute[] attributes)
 		{
 			if (value is SizeF)

src/Mono.Android/Xamarin.Android.Net/AndroidClientHandler.cs

@@ -313,14 +313,9 @@ protected virtual Task SetupRequest (HttpRequestMessage request, HttpURLConnecti
 		object? GetUnderlyingHandler ()
 		{
 			var fieldName = "_nativeHandler";
-			FieldInfo? field = null;
-
-			for (var type = GetType (); type != null; type = type.BaseType) {
-				field = type.GetField (fieldName, BindingFlags.Instance | BindingFlags.NonPublic);
-				if (field != null)
-					break;
-			}
-
+			const BindingFlags flags = BindingFlags.Instance | BindingFlags.NonPublic;
+			FieldInfo? field = typeof (HttpClientHandler).GetField (fieldName, flags) ??
+				typeof (HttpMessageHandler).GetField (fieldName, flags);
 			if (field == null) {
 				throw new InvalidOperationException ($"Field '{fieldName}' is missing from type '{GetType ()}'.");
 			}