Generalize @Keep annotation Proguard rules

Consolidate the Proguard rules for @Keep annotations for:
  * android.support.annotation.Keep (existing)
  * androidx.annotation.Keep (existing)
  * com.android.internal.annotation.Keep (new)

This ensures consistent semantics for clients using these annotations
across the codebase. Ideally there would be a single annotation, but
the support libraries are not available everywhere. In practice, this
change should be a no-op.

Bug: 210510433
Test: m + validate identical artifact sizes
Change-Id: Iab494e03269e5758d7f860331e971826ecc08cdd