Fixes multiple activity snapshots can stay in cache.

1. Fix snaphsot isn't remove from cache because there didn't search
the activity outside of transition. It's a regression from ag/23898517
2. Skip record activity snapshot if the persist queue is too deep.
The StoreWriteQueueItem will hold a snapshot reference and waiting
to write on disk in perisist queue, skip record the snapshot if there
already have too many store items, since those will be purge anyway.

Bug: 382014256
Flag: EXEMPT bugfix
Test: atest ActivitySnapshotControllerTests
Test: Launch activity continuously like stress test, monitor the
activity snapshot won't increase repidly within a short of time, and
there should always only one snapshot left in activity snapshot cache.

Change-Id: I65b245713682bc7796eb132050bc5bcebb0972dd