Fixes: #8421
Context: dotnet/java-interop#719
Context: #5278

In dotnet/java-interop#719, we realized we needed to call GC.KeepAlive () on objects that are passed to native code to prevent the GC from collecting them prematurely.

However, this only solved the issue for newly compiled binding libraries and not the existing ecosystem of binding libraries. Thus we added #5278, a "linker" step that would add GC.KeepAlive () calls to existing binding libraries.

The AddKeepAlivesStep runs on all assemblies the contain a reference to Java.Lang.Object, including ones that already have the keep-alives compiled in. Modify the step to skip assemblies that are built against .NET for Android, as we know they have the compiled keep-alives. Technically we could go back to Xamarin.Android 11.1, but using .NET for Android as the cutoff will easily filter out the vast majority of binding libraries.

This results in decent savings for the following test case:

dotnet new android
dotnet add package Xamarin.AndroidX.Activity --version 1.9.3.1
dotnet build -p:AndroidAddKeepAlives=true

However, in cases like #8421 where this step seems to be taking an abnormal amount of time there should be substantial improvements.