Check isUserUnlocked before attemp to delete snapshot file.

On different platform the first launched task may not be home task,
so after the first task removed, TasSnapshotController will ask to
remove it's snapshot files. But user may haven't unlock at that time.
To ensure the delete action happen after user unlock, also check
isUserUnlocked in DeleteWriteQueueItem#isReady.

Also dumps items in SnapshotPersistQueue#mWriteQueue for further
debugging if needed.

Bug: 321587472
Test: presubmit
Change-Id: Ia13008b13cfde4f831be8da29230179ac2c35374