Release sleep wake lock if activities are stopped while sleeping

The activity slept callback was removed since commit 4679918c. So
now just add the check at when an activity is stopped. Otherwise
it always takes 5s timeout to release the sleep wake lock.

Bug: 173186282
Test: ActivityTaskManagerServiceTests#testUpdateSleep

Change-Id: I7e662fea0c01ada95951009682375cd9fcec0b01