Make GetCustomAttribute calls optional in JniTypeManager

Optionally do not look for the `JniAddNativeMethodRegistrationAttribute`.
It turns out that this attribute is not commonly present in
Xamarin.Android applications and so all of the calls to
`GetCustomAttribute` failed to find it, incurring unnecessary
performance penalty on the application.

Make the code consult a global flag (in case of Xamarin.Android it is
set on the build time) which tells it whether or not to skip querying
for the above custom attribute. The flag is `true` by default to
maintain backward compatibility.

Author: grendello

src/Java.Interop/Java.Interop/JniRuntime.JniTypeManager.cs

@@ -333,6 +333,9 @@ static bool TryRegisterNativeMembers (JniType nativeClass, Type marshalType, str
 
 			static bool FindAndCallRegisterMethod (Type marshalType, JniNativeMethodRegistrationArguments arguments)
 			{
+				if (!JniTypeManagerOptions.JniAddNativeMethodRegistrationAttributePresent)
+					return false;
+
 				bool found = false;
 
 				foreach (var methodInfo in marshalType.GetRuntimeMethods ()) {

src/Java.Interop/Java.Interop/JniTypeManagerOptions.cs

@@ -0,0 +1,10 @@
+namespace Java.Interop
+{
+	// These settings affect all instances of JniTypeManager
+	public static class JniTypeManagerOptions
+	{
+		// If `false`, reflection checks for the presence of the `JniAddNativeMethodRegistrationAttribute` attribute
+		// throughout the **entire** application will be skipped.
+		public static bool JniAddNativeMethodRegistrationAttributePresent { get; set; } = true;
+	}
+}