Call thread join before tearing down

Since mSSTTestHandler may be in the middle of handling a message when
quit() is called, we call mSSTTestHandler.join() to ensure that the
thread has finished execution before tearing down objects.

Change-Id: Iee9193836dd93991209055fdbbf7cda9bbf91267
Fixes: 78357839
Test: atest FrameworksTelephonyTests#ServiceStateTrackerTests