[Xamarin.Android.Build.Tasks] Crash when using NetStandard and Xamarin.Forms

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

Commit 8f2ae24 attempted to fix this issue. However it made the
assumption that `@(ReferenceCopyLocalPaths)` contained the same
items as `@(ReferencePath)`. However it did not. So we should use
a combination of the two to figure out what assmeblies are needed.
We need to disgard reference assemblies though, we do this by
checking for the appropriate attributes before we add the assembly
to the list of items we want to package.

Author: dellis1972

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

@@ -76,6 +76,10 @@ bool Execute (DirectoryAssemblyResolver resolver)
 					var assemblyDef = resolver.Load (assembly.ItemSpec);
 					if (assemblyDef == null)
 						throw new InvalidOperationException ("Failed to load assembly " + assembly.ItemSpec);
+					if (MonoAndroidHelper.IsReferenceAssembly (assemblyDef)) {
+						Log.LogWarning ($"Ignoring {assembly_path} as it is a Reference Assembly");
+						continue;
+					}
 					topAssemblyReferences.Add (assemblyDef);
 					assemblies.Add (Path.GetFullPath (assemblyDef.MainModule.FullyQualifiedName));
 				}

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

@@ -303,10 +303,15 @@ public static bool IsFrameworkAssembly (string assembly, bool checkSdkPath)
 
 		public static bool IsReferenceAssembly (string assembly)
 		{
-			var a = AssemblyDefinition.ReadAssembly (assembly, new ReaderParameters() { InMemory = true, ReadSymbols = false, });
-			if (!a.HasCustomAttributes)
+			var a = AssemblyDefinition.ReadAssembly (assembly, new ReaderParameters () { InMemory = true, ReadSymbols = false, });
+			return IsReferenceAssembly (a);
+		}
+
+		public static bool IsReferenceAssembly (AssemblyDefinition assembly)
+		{
+			if (!assembly.HasCustomAttributes)
 				return false;
-			return a.CustomAttributes.Any (t => t.AttributeType.FullName == "System.Runtime.CompilerServices.ReferenceAssemblyAttribute");
+			return assembly.CustomAttributes.Any (t => t.AttributeType.FullName == "System.Runtime.CompilerServices.ReferenceAssemblyAttribute");
 		}
 
 		public static bool ExistsInFrameworkPath (string assembly)

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

@@ -1492,10 +1492,9 @@ because xbuild doesn't support framework reference assemblies.
 		<FilteredAssemblies Include="$(OutDir)$(TargetFileName)"
 				Condition="Exists ('$(OutDir)$(TargetFileName)')" />
 		<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 -->
+				Condition="'%(ReferenceCopyLocalPaths.ResolvedFrom)' != 'ImplicitlyExpandDesignTimeFacades' And '%(ReferenceCopyLocalPaths.Extension)' == '.dll' "/>
 		<FilteredAssemblies Include="%(ReferencePath.Identity)"
-				Condition="'%(ReferencePath.ResolvedFrom)' != 'ImplicitlyExpandDesignTimeFacades' And '@(ReferenceCopyLocalPaths)' == '' "/>
+				Condition="'%(ReferencePath.ResolvedFrom)' != 'ImplicitlyExpandDesignTimeFacades' "/>
 	</ItemGroup>
 	<!-- Find all the assemblies this app requires -->
 	<ResolveAssemblies