Further refine VisibleForTesting rules

Additional analysis revealed more edge cases where external library
code was getting kept unnecessarily. Further refine the keep rules to
avoid these cases. Now, we only apply keep globally for the
`com.android.internal.annotations.VisibleForTesting` annotation.

VisibleForTesting annotations defined externally (e.g., by androidx or
guava) will *only* be respected in platform-defined packages. This
helps trim unused code from deps like gRPC and Dagger, saving up to
~hundreds of KB per package that uses these libraries.

Bug: 248580093
Test: m + verify exclusion of external test code (e.g., from dagger)
Change-Id: Iab7559c08d3ae1ac74f18e3cf3a1b4828a3736cf