DO NOT MERGE Add stop method to backup handler thread.

Currently we call .quit() on the underlying thread which will cause all
messages to stop being processed. This has the side effect that, because
the backup system is a state machine where the state transitions are
messages, the message to transition into a state where the WakeLock is
released may not occur when a user is torn down.

This change adds a stop method we can call instead of .quit() on the
thread which drops any remaining messages and then releases the
WakeLock.

We also wrap the wakelock acquire/release calls to prevent any acquire/release on
the underlying wakelock after a quit. For the acquire, this avoids a non-released
wakelock and for the release, this avoids a runtime exception which can happen
when we release a released wakelock

Test: atest CtsBackupTestCases CtsBackupHostTestCases
Test: m RunBackupFrameworksServicesRoboTests && atest RunBackupFrameworksServicesRoboTests
Test: blaze run -- //experimental/users/nathch/py/bug_repros:repro 136264323 -m acquire_quit  -log DEBUG
Test: blaze run -- //experimental/users/nathch/py/bug_repros:repro 136264323 -m quit_acquire  -log DEBUG
Test: blaze run -- //experimental/users/nathch/py/bug_repros:repro 136264323 -m acquire_quit_release  -log DEBUG
Bug: 136264323

Change-Id: I42dcf997fc44cde05695a563aa19c8e47f6f9f26