[build] improve Xamarin.Android.sln usability in VS Windows

[jonathanpeppers]

Xamarin.Android.sln has two issues that make Xamarin.Android.sln not
work too well in VS Windows:

• It builds MSBuild tasks and uses them within the same solution, such
  as xa-prep-tasks.csproj and
  Xamarin.Android.Tools.BootstrapTasks.csproj.
• We have conditional @(ProjectReference)

To improve things, I completely removed the $(XAIntegratedTests)
MSBuild property and any @(ProjectReference) that were conditional.

I moved the two bootstrap .csproj files to a new
Xamarin.Android.BootstrapTasks.sln which gets built during make prepare or msbuild Xamarin.Android.sln /t:Prepare.

VS Windows was also complaining about the
CreateManifestResourceNames target missing for projects that merely
import Microsoft.Common.targets such as r8.csproj. I added an
empty target in Configuration.props to fix all cases of this.

Unfortunately, things are still not perfect as you can hit errors such
as:

Unable to copy file "obj\Debug\Xamarin.Android.Build.Tasks.dll" to "C:\src\xamarin-android\bin\Debug\lib\xamarin.android\xbuild\Xamarin\Android\Xamarin.Android.Build.Tasks.dll".
The process cannot access the file 'C:\src\xamarin-android\bin\Debug\lib\xamarin.android\xbuild\Xamarin\Android\Xamarin.Android.Build.Tasks.dll' because it is being used by another process.
Could not copy "obj\Debug\Xamarin.Android.Build.Tasks.dll" to "C:\src\xamarin-android\bin\Debug\lib\xamarin.android\xbuild\Xamarin\Android\Xamarin.Android.Build.Tasks.dll".
Exceeded retry count of 10. Failed. The file is locked by: "Microsoft Visual Studio 2019 (26660), MSBuild.exe (18800), MSBuild.exe (17416)"	Xamarin.Android.Build.Tasks

This occurs if you modify Xamarin.Android.Build.Tasks.csproj.

I am not yet sure if we can get around this remaining problem, but we
can revisit in the future.

[jonpryor]

Do we have an explanation for the file sharing issue on Xamarin.Android.Build.Tasks.dll? Why is it being held open?

[jonathanpeppers]

VS Windows keeps MSBuild nodes alive, and if they have previously loaded Xamarin.Android.Build.Tasks.dll the assembly is locked.

At least under .NET framework, it seems like they are just using Assembly.LoadFrom in MSBuild: https://github.com/microsoft/msbuild/search?q=assembly.loadfrom

[jonpryor]

Why do we need this change on Windows?

[jonathanpeppers]

This PR removes the "bootstrap" MSBuild tasks from the SLN, so we need make prepare to build them now.

I think it makes sense to build them at that step? or do you think make all should do that?

[jonpryor]

I think we need to do the "same" thing in Java.Interop -- not that I'm asking @jonathanpeppers to do so -- as that may help the VSMac experience. (I think it'll also be needed to land PR #3393...)