Refactor weakly referenced callback keep rules

Annotate callback types that use weak reference semantics with a new
@WeaklyReferencedCallback annotation. Apply keep rules to fields that
implement types with this annotation, ensuring such fields are kept
during optimization and can provide a strong ref anchor at runtime.

Note that we only do this *only* on SysUISingleton-annotated classes,
as all other types should explicitly unregister any registered
callbacks, and aren't subject to fields being optimized away if only
referenced in the constructor.

This saves ~40-50s from the SystemUI R8 optimization pass.

Test: time m SystemUI + inspect dex diff
Bug: 300478836
Bug: 264686688
Change-Id: I6ca8b60460991c12436841a275e180beaf16cdb8