[Xamarin.Android.Build.Tasks] Fix native code generation when marshal methods are disabled (dotnet#7899)

Disabling marshal method generation via
`$(AndroidEnableMarshalMethods)`=False turns off a lot of native code
generation in the `MarshalMethodsNativeAssemblyGenerator` class, but
the class is also responsible for outputting a correctly sized cache
area (an array of *X* pointers) to be used by the native runtime to
cache pointers to `MonoImage` instances.

`libmonodroid.so` trusts that `Xamarin.Android.Build.Tasks.dll` et al
will generate correct code, and thus does not verify the size of
generated array.  This trust, unfortunately, was broken because with
marshal methods disabled, the native code generator created an output
cache array that was 0 entries in size, thus leading to segfault when
attempting to run the application.

Fix the issue and also parametrize one of the on-device tests to be
built twice, with marshal methods explicitly disabled and explicitly
enabled.

Author: grendello

src/Xamarin.Android.Build.Tasks/Tasks/GeneratePackageManagerJava.cs

@@ -413,7 +413,7 @@ void AddEnvironment ()
 					Log
 				);
 			} else {
-				marshalMethodsAsmGen = new MarshalMethodsNativeAssemblyGenerator (uniqueAssemblyNames);
+				marshalMethodsAsmGen = new MarshalMethodsNativeAssemblyGenerator (assemblyCount, uniqueAssemblyNames);
 			}
 			marshalMethodsAsmGen.Init ();
 

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Android/KnownProperties.cs

@@ -12,6 +12,7 @@ public static class KnownProperties
 		public const string AndroidUseLatestPlatformSdk = "AndroidUseLatestPlatformSdk";
 		public const string AndroidUseAapt2 = "AndroidUseAapt2";
 		public const string AndroidCreatePackagePerAbi = "AndroidCreatePackagePerAbi";
+		public const string AndroidEnableMarshalMethods = "AndroidEnableMarshalMethods";
 
 		public const string AndroidSupportedAbis = "AndroidSupportedAbis";
 		public const string RuntimeIdentifier = "RuntimeIdentifier";

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Android/XamarinAndroidApplicationProject.cs

@@ -166,6 +166,11 @@ public AndroidLinkMode AndroidLinkModeRelease {
 			set { SetProperty (ReleaseProperties, KnownProperties.AndroidLinkMode, value.ToString ()); }
 		}
 
+		public bool EnableMarshalMethods {
+			get { return string.Equals (GetProperty (KnownProperties.AndroidEnableMarshalMethods), "True", StringComparison.OrdinalIgnoreCase); }
+			set { SetProperty (KnownProperties.AndroidEnableMarshalMethods, value.ToString ()); }
+		}
+
 		public string AndroidManifest { get; set; }
 		public string LayoutMain { get; set; }
 		public string MainActivity { get; set; }

src/Xamarin.Android.Build.Tasks/Utilities/MarshalMethodsNativeAssemblyGenerator.cs

@@ -204,8 +204,9 @@ sealed class MarshalMethodName
 		/// <summary>
 		/// Constructor to be used ONLY when marshal methods are DISABLED
 		/// </summary>
-		public MarshalMethodsNativeAssemblyGenerator (ICollection<string> uniqueAssemblyNames)
+		public MarshalMethodsNativeAssemblyGenerator (int numberOfAssembliesInApk, ICollection<string> uniqueAssemblyNames)
 		{
+			this.numberOfAssembliesInApk = numberOfAssembliesInApk;
 			this.uniqueAssemblyNames = uniqueAssemblyNames ?? throw new ArgumentNullException (nameof (uniqueAssemblyNames));
 			generateEmptyCode = true;
 		}

tests/MSBuildDeviceIntegration/Tests/InstallAndRunTests.cs

@@ -26,7 +26,7 @@ public void Teardown ()
 		}
 
 		[Test]
-		public void NativeAssemblyCacheWithSatelliteAssemblies ()
+		public void NativeAssemblyCacheWithSatelliteAssemblies ([Values (true, false)] bool enableMarshalMethods)
 		{
 			var path = Path.Combine ("temp", TestName);
 			var lib = new XamarinAndroidLibraryProject {
@@ -49,6 +49,7 @@ public void NativeAssemblyCacheWithSatelliteAssemblies ()
 
 			proj = new XamarinAndroidApplicationProject {
 				IsRelease = true,
+				EnableMarshalMethods = enableMarshalMethods,
 			};
 			proj.References.Add (new BuildItem.ProjectReference ($"..\\{lib.ProjectName}\\{lib.ProjectName}.csproj", lib.ProjectName, lib.ProjectGuid));
 			proj.SetAndroidSupportedAbis ("armeabi-v7a", "arm64-v8a", "x86", "x86_64");