[Microsoft.Android.Build] Use MSBuild NuGets

[pjcollins]

I've been hitting errors running tests locally on Windows after building
with .NET 6 Preview 3:

Error Message:
 System.IO.FileNotFoundException : Could not load file or assembly 'System.Runtime, Version=5.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'. The system cannot find the file specified.
Stack Trace:
   at Xamarin.ProjectTools.DotNetXamarinProject.SaveProject()
 at Xamarin.ProjectTools.XamarinProject.Save(Boolean saveProject) in C:\Users\Peter\source\xamarin-android\src\Xamarin.Android.Build.Tasks\Tests\Xamarin.ProjectTools\Common\XamarinProject.cs:line 241
 at Xamarin.ProjectTools.ProjectBuilder.Save(XamarinProject project, Boolean doNotCleanupOnUpdate, Boolean saveProject) in C:\Users\Peter\source\xamarin-android\src\Xamarin.Android.Build.Tasks\Tests\Xamarin.ProjectTools\Common\ProjectBuilder.cs:line 43
 at Xamarin.ProjectTools.ProjectBuilder.Build(XamarinProject project, Boolean doNotCleanupOnUpdate, String[] parameters, Boolean saveProject, Dictionary`2 environmentVariables) in C:\Users\Peter\source\xamarin-android\src\Xamarin.Android.Build.Tasks\Tests\Xamarin.ProjectTools\Common\ProjectBuilder.cs:line 67
 at Xamarin.Android.Build.Tests.WearTests.ResolveLibraryImportsWithReadonlyFiles() in C:\Users\Peter\source\xamarin-android\src\Xamarin.Android.Build.Tasks\Tests\Xamarin.Android.Build.Tests\WearTests.cs:line 28

Rather than copying the version of the MSBuild assemblies used to build
our test projects to the output directory, we should be able to use the
NuGet package content directly. These files should only be used by our
project creation APIs and as a result their version shouldn't matter.

[jonathanpeppers]

Can we run xamarin-android's CI to see if this breaks anything? Otherwise looks good to me.

[jonathanpeppers]

Ok, this looks good if the build works out here:

dotnet/android#5891

[jonathanpeppers]

I'm going to merge this, as it looks like .NET 6 installs are affecting other PRs:

https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=4707308&view=ms.vss-test-web.build-test-results-tab&runId=21262996&resultId=100031&paneView=debug