Tuning snapshot persist queue for shell transition.

- Remove duplicate write snapshot to files request, there could record
  snapshot for a same task more than one time during a transition, but
  only the latest request matter when save files.
- Insert load snapshot request in front of queue,so it can happen
  before write request. Because back navigation might need that
  snapshot as soon as previous transition finish.

Bug: 259497289
Test: atest TaskSnapshotPersisterLoaderTest TaskSnapshotCacheTest\
TaskSnapshotLowResDisabledTest

Change-Id: Ib5ff76e38578bf0b8c5baf796329da8ac72160e2