Context: https://github.com/dotnet/designs/blob/38bffe6c03a149c80f1c3b22787f33c794e71c76/accepted/2021/net6.0-tfms/net6.0-tfms.md

We don't have a complete implementation for .NET 6 & NuGet yet, so for
for things to work *at all* we have `Microsoft.Android.Sdk.NuGet.targets`
that contains workarounds that enable:

* Given some NuGet package, `Foo.nupkg`
* Try `lib/net6.0-android/Foo.dll`
* Try `lib/monoandroid10.0/Foo.dll`, other `$(TFV)` etc.

We can now test the above scenario with AndroidX NuGet packages:

* dotnet/android-libraries#247
* https://pkgs.dev.azure.com/azure-public/vside/_packaging/xamarin-impl/nuget/v3/index.json
* `<PackageReference Include="Xamarin.AndroidX.AppCompat" Version = "1.2.0.7-net6preview01" />`

Unfortunately, consuming `Xamarin.AndroidX.AppCompat` results in our
build using `lib/monoandroid90/Xamarin.AndroidX.AppCompat.dll` over
the .NET 6 one!

I was able to reproduce this failure in the `XASdkTests.DotNetBuild()`
test, as any `Release` build will fail during linking when using
`MonoAndroid` assemblies without `KnownPackages.AddDotNetCompatPackages()`.

To solve the problem, we need to list `net6.0-android30.0` in
`$(PackageTargetFallback)`. Now .NET 6 packages are preferred over
`MonoAndroid` ones. Lastly I added `MonoAndroid11.0`, because it was
missing.

Eventually I hope to delete `Microsoft.Android.Sdk.NuGet.targets`
completely when the NuGet implementation of TFMs in .NET 6 is
complete.