Add keep rules for @KeepForWeakReference annotation

Ensure annotated fields are kept when treeshaking-related optimizations
are enabled. Note that this still allows field removal if tree shaking
reveals no references, but will avoid removal if there are references
that might impact downstream usage, e.g., weak references.

Bug: 227455445
Test: atest InternalAnnotationsTest
Change-Id: I59c6277c43b14c5de8154fa1eefad01f0702e413