Clean up KeepForWeakReference usage

We now rely on the WeaklyReferencedCallback annotation to automatically
preserve fields that are needed to keep callback type instances that are
weakly referenced when registered.

The affected members should be identically preserved with this change.

Bug: 349245577
Test: FULL_SYSTEM_OPTIMIZE_JAVA=true m services
Flag: EXEMPT refactor
Change-Id: I67cad0c66d966c43b269738b62063ce5b0c1270e