Use targeting pack metadata to set $(_XATargetFrameworkDirectories)

Certain tasks require that $(_XATargetFrameworkDirectories) contains
paths to both Mono.Android.dll and mscorlib.dll. In order to ensure that
this property contains all known assembly paths we'll use the
@(ResolvedTargetingPack) ItemGroup. These items contain metadata
definitions for %(TargetFramework) and %(PackagePath), which can be
combined to construct the full assembly path for each pack.

Author: pjcollins

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

@@ -50,12 +50,16 @@ projects.
   </PropertyGroup>
 
   <Target Name="_ResolveSdks" DependsOnTargets="$(_ResolveSdksDependsOnTargets)">
+    <!-- When using .NET 5, provide a list of paths to the Android and NETCore targeting pack directories, e.g.
+          `packages\microsoft.android.ref\10.0.100-ci.master.22\ref\net5.0\` and
+          `C:\Program Files\dotnet\packs\Microsoft.NETCore.App.Ref\5.0.0-preview.6.20264.1\ref\net5.0\`
+    -->
+    <ItemGroup Condition=" '$(UsingAndroidNETSdk)' == 'True' ">
+      <_ResolveSdksFrameworkRefAssemblyPaths Include="@(ResolvedTargetingPack->'%(PackageDirectory)\ref\%(TargetFramework)')" Condition=" '@(ResolvedTargetingPack)' == '@(FrameworkReference)' and '%(Identity)' != '' " />
+    </ItemGroup>
     <PropertyGroup>
       <_AndroidAllowMissingSdkTooling Condition=" '$(_AndroidAllowMissingSdkTooling)' == '' ">False</_AndroidAllowMissingSdkTooling>
-      <!-- When using .NET 5, provide the path to Mono.Android.dll e.g.
-            `packages\microsoft.android.ref\10.0.100-ci.master.22\ref\net5.0\`
-      -->
-      <_XATargetFrameworkDirectories Condition=" '$(UsingAndroidNETSdk)' == 'True' And '%(FileName)%(Extension)' == 'Mono.Android.dll'  ">@(Reference->'%(RootDir)%(Directory)')</_XATargetFrameworkDirectories>
+      <_XATargetFrameworkDirectories Condition=" '$(UsingAndroidNETSdk)' == 'True' ">@(_ResolveSdksFrameworkRefAssemblyPaths)</_XATargetFrameworkDirectories>
     </PropertyGroup>
     <ResolveSdks
         ContinueOnError="$(_AndroidAllowMissingSdkTooling)"