Move sleep from ActivityStack to Supervisor

- Move activitySleptLocked, checkReadyForSleepLocked,
mGoingToSleepActivities, mGoingToSleep, mSleepTimeout from
ActivityStack to ActivityStackSupervisor.

- Fix bug that was causing mGoingToSleepActivities to always be
empty.

- Add more debug logging.

Change-Id: Ibd57bd49bf6f468567571b390f3051faaeee1aa1