Keep members needed for downstream weak refs

Annotate several fields that are needed solely to keep alive downstream
weak refs. A more general solution is being pursued, but for now this
addresses some of the known issues when full optimization is enabled.

Test: FULL_SYSTEM_OPTIMIZE_JAVA=true m + check member preserved
Bug: 349245577
Flag: EXEMPT bugfix
Change-Id: I3683fd9909cd7881a907a26b8dcd564d1877df64