[coreclr] Enable R2R builds

[ivanpovazan]

Description

ReadyToRun deployments are enabled when:

1. <PublishReadyToRun>true</PublishReadyToRun> is added to the project file
2. App is published via: dotnet publish

Once enabled, the SDK injects necessary build tasks to perform AOT compilation of assemblies after their trimming and creation of R2R images during app publish.

Problem

Currently, Android app build runs two assembly postprocessing build tasks (after R2R image generation): RewriteMarshalMethods and RemoveRegisterAttribute that are modifying assemblies with Mono.Cecil. However, altering R2R assemblies is not supported with Mono.Cecil and build fails via:

System.NotSupportedException: Writing mixed-mode assemblies is not supported
         at Mono.Cecil.ModuleWriter.Write(ModuleDefinition module, Disposable`1 stream, WriterParameters parameters)
         at Mono.Cecil.ModuleWriter.WriteModule(ModuleDefinition module, Disposable`1 stream, WriterParameters parameters)
         at Mono.Cecil.ModuleDefinition.Write(Stream stream, WriterParameters parameters)
         at Mono.Cecil.ModuleDefinition.Write(WriterParameters parameters)
         at Mono.Cecil.ModuleDefinition.Write()
         at Mono.Cecil.AssemblyDefinition.Write()
         at Xamarin.Android.Tasks.RemoveRegisterAttribute.RunTask() in /Users/runner/work/1/s/xamarin-android/src/Xamarin.Android.Build.Tasks/Tasks/RemoveRegisterAtt
      ribute.cs:line 37
         at Microsoft.Android.Build.Tasks.AndroidTask.Execute() in /Users/runner/work/1/s/xamarin-android/external/xamarin-android-tools/src/Microsoft.Android.Build.
      BaseTasks/AndroidTask.cs:line 25

Changes

To enable R2R builds, this PR:

• disables running RewriteMarshalMethods and RemoveRegisterAttribute build tasks in ReadyToRun deployments
• adds new unit tests for R2R builds
• reenables unit tests for regular CoreCLR builds

Future work

Investigate running R2R image generation build task after RewriteMarshalMethods and RemoveRegisterAttribute so that R2R builds could also benefit from the performance improvements these build tasks bring.

[grendello]

I think I'd rather wait with merging this PR until marshal methods can be used again. Maybe it's a simple matter of ordering? R2R should be called, like Mono AOT, after we're done modifying the assemblies.