[Xamarin.Android.Build.Tasks] remove @(_AndroidAssemblySkipCases)

[jonathanpeppers]

In an effort to remove old, unused code, we should be able to remove the following related to the old Android.Support.* libraries:

• Xamarin.Android.SkipCases.projitems and the @(_AndroidAssemblySkipCases) item group.

This was a build performance optimization added in 02c07ed that would skip a build step that normalized casing for .dll files with an embedded __AndroidLibraryProjects__.zip. This optimization is not needed at all for .aar files.

In .NET 9 projects, we don't create or embed __AndroidLibraryProjects__.zip, so modern projects don't need this optimization. Instead we bundle a MyLibrary.aar alongside MyLibrary.dll inside of a MyLibrary NuGet package.

• Automatic trimming of the Xamarin.Android.Support.* assemblies

Modern packages mark themselves trimmable with $(IsTrimmable) or the assembly-level attribute. Let's just remove the case for Android.Support here.

[jpobst]

We've been marking all of our packages as <IsTrimmable> for nearly 2 years now, so we can probably remove all of the hardcoding there if we want.

• Mark all assemblies as Trimmable via Directory.Build.props. android-libraries#520
• net6 trimmable support xamarin/GooglePlayServicesComponents#597

[jonathanpeppers]

We've been marking all of our packages as <IsTrimmable> for nearly 2 years now, so we can probably remove all of the hardcoding there if we want.

I will try it at some point, but there are some Xamarin.Forms app size tests that will probably fail:

https://github.com/xamarin/xamarin-android/blob/af7c12c7cd0a77e5024da4c60aad903984e855fa/tests/Xamarin.Forms-Performance-Integration/Xamarin.Forms.Performance.Integration.csproj#L13

We might need to move this test to Xamarin.Forms 5.x, as I bet this project's dependencies are ancient.