Tweak WeaklyReferencedCallback keep rules

Adjust the semantics so that we only prevent optimization, and we avoid
unnecessary keeps for synthetically generated members.

This should have no effective impact on weakly referenced lifecycle
semantics, though the generated dex code might be slightly different.

Bug: 349245577
Test: FULL_SYSTEM_OPTIMIZE_JAVA=true m services
Flag: EXEMPT refactor
Change-Id: I7a6de2d8db593a5336f94448f18a75482e4ed349