Add keep rules for WeaklyReferencedCallback annotation

Ensure fields of types with this annotation are kept, preserving
lifecycle semantics during bytecode optimization.

A follow-up change will refactor these global rules, moving
rules specific to the framework-annotations-lib to that
target, where they should now be properly inherited by downstream
optimized targets.

Bug: 349245577
Test: atest InternalAnnotationsTests
Flag: EXEMPT bugfix
Change-Id: Iaf7694eea3d8de05a007e4c8c9e98c3eb58098ae