[.NET 5] NuGet workarounds for Xamarin.Forms projects

[jonathanpeppers]

Context: https://github.com/xamarin/net5-samples/blob/d525d8a2d60700f52f86372c47e83d646d800aa4/Directory.Android.targets

Currently, .NET 5 will not restore existing Xamarin.Android NuGet
packages. It does not know to map netcoreapp5.0 to
MonoAndroid10.0, MonoAndroid9 etc.

We'll need to work around this until this lands:

• add support for net5.0-<platform> NuGet/NuGet.Client#3339

To get a Xamarin.Forms project building & running at all in
xamarin/net5-samples, I had to:

1. Change $(AssetTargetFallback)
2. Write an MSBuild target to replace any instances of
   netstandard2.0\Xamarin.Forms.Platform.dll with
   MonoAndroid10.0\Xamarin.Forms.Platform.dll.
3. Add additional platform-specific assemblies in
   $(PkgXamarin_Forms)\lib\MonoAndroid10.0\

There are still some issues with this:

1. You get a lot of NU1701 warnings due to $(AssetTargetFallback).
2. Due to: https://github.com/NuGet/docs.microsoft.com-nuget/issues/1955
   You have to manually list every transitive dependency, which
   totals around ~36 packages for AndroidX.

These drawbacks make it completely impractical to run some subset of
our existing MSBuild tests on .NET 5. We would have to list the entire
tree of <PackageReference/> for every test.

Since we might be waiting a bit of time for this, I have been able to
come up with some workarounds to solve these problems for now:

1. Add a Microsoft.Android.Sdk.NuGet.targets with workarounds that
   we will completely remove down the road (I hope!).
2. Use $(PackageTargetFallback) instead of $(AssetTargetFallback).
   It does not emit NU1701 warnings, and transitive dependencies
   work! It is a deprecated MSBuild property, but should be fine as a
   workaround.
3. Write a _FixupNuGetReferences MSBuild target that runs after
   ResolvePackageAssets. A <FixupNuGetReferences/> MSBuild task
   will remove any netstandard2.0 assemblies where platform-specific
   ones exist. It also needs to add platform-specific assemblies that
   are missing.

With this in place, a Xamarin.Forms .csproj with no hacks works!

 <Project Sdk="Microsoft.Android.Sdk">
   <PropertyGroup>
    <TargetFramework>netcoreapp5.0</TargetFramework>
    <OutputType>Exe</OutputType>
   </PropertyGroup>
   <ItemGroup>
    <PackageReference Include="Xamarin.Forms" Version="4.5.0.617" />
   </ItemGroup>
 </Project>

I created a new XamarinFormsXASdkProject class for use in our
existing .NET 5 MSBuild tests.

Other changes:

• $(XamarinAndroidVersion) needs to be filled out for the AndroidX
  MSBuild targets:
  https://github.com/xamarin/XamarinAndroidXMigration/blob/ea130ca0d0e9b3e20edccec02364f36da11ada6b/source/Xamarin.AndroidX.Migration/BuildTasks/Xamarin.AndroidX.Migration.targets#L107
• $(RuntimeIdentifier) should have a default value for application
  projects.
• Some tweaks to make XASdkProject more flexible.
• Xamarin.ProjectTools.DotNetStandard projects now support
  <Import/>.
• Added AndroidX-compatible Tabbar.xml and Toolbar.xml

[dellis1972]

Looks ok. Made a comment about FixupNuGetReferences though.