Calling HandlerThread.quit() or .quitSafely() doesn't immediately terminate
the thread. It just instructs the Looper not to accept any new messages and
to terminate at the next opportunity. Added a HandlerThread.join() everywhere
where the intention is to close and release all resources and to stop all
threads.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=171525241
