DO NOT MERGE: Don't take the AM lock in cached app compaction handler

A single oomAdjUpdate pass could trigger quite a few app compactions,
each of them would need to acquire the AM lock during the handling,
results in lock contentions. Now use its own lock. Also skip the
scheduling if the given app is pending with compaction already.

Bug: 163297662
Test: atest CachedAppOptimizerTest
Test: Manual - Boot & check the logs
Change-Id: I4618a3ae78838cd9783a431e7364130270ebb4d6