Keep members that are weakly referenced from native code

When full optimization is enabled in system_server, members that are
write-only may be optimized away. This can impact downstream weak
reference lifecycle semantics. Annotate such a member in
LocalDisplayAdapter that is weakly referenced from native code.

Test: FULL_SYSTEM_OPTIMIZE_JAVA=true m + check member preserved
Bug: 349245577
Change-Id: I355ee0ff7009476d4b917fc8dd078922d1b2c15b