[TrimmableTypeMap] Generate ProGuard for types in type map

Part of https://github.com/dotnet/android/issues/10788

Once the list of types that survived trimming is known, we can generate ProGuard rules to specify which ACW/JCW Java classes we want to keep via the `-keep class jni.class.Name { *; }` rules.

- The build task needs to run _after_ ILLink or ILC and _before_ we run Java trimming (r8).
- This task should run incrementally _only_ when ILLink or ILC produce different typemap.
- This task should not run in Debug at all.

Getting the list of types that are in the typemap might not be trivial at the moment:

- CoreCLR: we can scan the trimmed type map assembly and see which attributes were kept
- Native AOT needs to give us the list of types it kept in the typemap: https://github.com/dotnet/runtime/issues/120204. As a workaround, we can opt into ILC's existing diagnostic outputs (`$(IlcGenerateMetadataLog)` or `$(IlcGenerateDgmlFile)`) but these artifacts are quite large and writing them to disk, reading them from disk, and scanning them will add non-trivial time to the overall build.

Ideally, ILLink and ILC will produce some form of _list_ of types which are included in the typemap, which would allow us to implement a unified build task to generate the proguard rules.