[Xamarin.Android.Build.Tasks] Don't package reference assemblies (dotnet#706)

Fixes: https://bugzilla.xamarin.com/show_bug.cgi?id=57342

The investigation in the bug suggests that we are picking up a
reference assembly rather than a the actual implementation.
This is to do with the way netstandard nuget packages work, they
include both `ref` and `lib` folders.

In this case `ref` was being included in the package rather than
`lib`. This commit alters the `_ResolveAssemblies` to use

	@(ReferenceCopyLocalPaths)

This ItemGroup is populated with the correct items.

Author: dellis1972

Makefile

@@ -149,7 +149,7 @@ TEST_APK_PROJECTS = \
 # Syntax: $(call BUILD_TEST_APK,path/to/project.csproj)
 define BUILD_TEST_APK
 	# Must use xabuild to ensure correct assemblies are resolved
-	MSBUILD="$(MSBUILD)" tools/scripts/xabuild /t:SignAndroidPackage $(1)
+	MSBUILD="$(MSBUILD)" tools/scripts/xabuild $(MSBUILD_FLAGS) /t:SignAndroidPackage $(1)
 endef	# BUILD_TEST_APK
 
 run-apk-tests:

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

@@ -273,6 +273,10 @@ private void AddAssemblies (ZipArchiveEx apk)
 
 			int count = 0;
 			foreach (ITaskItem assembly in ResolvedUserAssemblies) {
+
+				if (MonoAndroidHelper.IsReferenceAssembly (assembly.ItemSpec)) {
+					Log.LogWarning ($"{assembly.ItemSpec} is a reference assembly!");
+				}
 				// Add assembly
 				apk.Archive.AddFile (assembly.ItemSpec, GetTargetDirectory (assembly.ItemSpec) + "/"  + Path.GetFileName (assembly.ItemSpec), compressionMethod: CompressionMethod.Store);
 
@@ -305,6 +309,9 @@ private void AddAssemblies (ZipArchiveEx apk)
 			count = 0;
 			// Add framework assemblies
 			foreach (ITaskItem assembly in ResolvedFrameworkAssemblies) {
+				if (MonoAndroidHelper.IsReferenceAssembly (assembly.ItemSpec)) {
+					Log.LogWarning ($"{assembly.ItemSpec} is a reference assembly!");
+				}
 				apk.Archive.AddFile (assembly.ItemSpec, "assemblies/" + Path.GetFileName (assembly.ItemSpec), compressionMethod: CompressionMethod.Store);
 				var config = Path.ChangeExtension (assembly.ItemSpec, "dll.config");
 				AddAssemblyConfigEntry (apk, config);

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

@@ -8,6 +8,7 @@
 using Mono.Security.Cryptography;
 using Xamarin.Android.Build.Utilities;
 using Xamarin.Tools.Zip;
+using Mono.Cecil;
 
 #if MSBUILD
 using Microsoft.Build.Framework;
@@ -286,6 +287,14 @@ public static bool IsFrameworkAssembly (string assembly, bool checkSdkPath)
 			return TargetFrameworkDirectories == null || !checkSdkPath ? false : ExistsInFrameworkPath (assembly);
 		}
 
+		public static bool IsReferenceAssembly (string assembly)
+		{
+			var a = AssemblyDefinition.ReadAssembly (assembly, new ReaderParameters() { InMemory = true, ReadSymbols = false, });
+			if (!a.HasCustomAttributes)
+				return false;
+			return a.CustomAttributes.Any (t => t.AttributeType.FullName == "System.Runtime.CompilerServices.ReferenceAssemblyAttribute");
+		}
+
 		public static bool ExistsInFrameworkPath (string assembly)
 		{
 			return TargetFrameworkDirectories

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Common.targets

@@ -1417,7 +1417,11 @@ because xbuild doesn't support framework reference assemblies.
 <Target Name="_ResolveAssemblies">
 	<!--- Remove the ImplicitlyExpandDesignTimeFacades assemblies. We have already build the app there are not required for packaging  -->
 	<ItemGroup>
-		<FilteredAssemblies Include="@(ReferencePath)" Condition="'%(ReferencePath.ResolvedFrom)' != 'ImplicitlyExpandDesignTimeFacades'" />
+		<FilteredAssemblies Include="%(ReferenceCopyLocalPaths.Identity)"
+				Condition="'%(ReferenceCopyLocalPaths.ResolvedFrom)' != 'ImplicitlyExpandDesignTimeFacades' And '%(ReferenceCopyLocalPaths.Extension)' == '.dll' And '%(ReferenceCopyLocalPaths.DestinationSubDirectory)' == '' "/>
+		<!-- Fallback to @(ReferencePath) if @(ReferenceCopyLocalPaths) is empty. This is for xbuild support -->
+		<FilteredAssemblies Include="%(ReferencePath.Identity)"
+				Condition="'%(ReferencePath.ResolvedFrom)' != 'ImplicitlyExpandDesignTimeFacades' And '@(ReferenceCopyLocalPaths)' == '' "/>
 	</ItemGroup>
 	<!-- Find all the assemblies this app requires -->
 	<ResolveAssemblies