Mark all Android runtime pack assemblies as trimmable

[marek-safar]

All Android assemblies should be marked as trimmable and code which tries to override the settings should be removed.

This is the preferred way to tell linker to include assemblies in the trimming process in .NET6 P3

[jonathanpeppers]

I think we already have this:

https://github.com/xamarin/xamarin-android/blob/72da34928f3ed0abce5e0c0db313fbf54b428b58/build-tools/create-packs/Microsoft.Android.Sdk.proj#L107-L119

We have IsTrimmable="true" on the @(KnownFrameworkReference), or do we also need something else? Thanks.

[marek-safar]

As explained in the linked design doc

Instead of using IsTrimmable metadata in the SDK to control trimmable assemblies, we will move to a model where all trimmable SDK assemblies are built with [assembly: AssemblyMetadata("IsTrimmable", "True")].