aaudio: cleanup thread handling

Fix problem when an attempt is made to join the callback from the
callback. It used to leave mHasThread in the wrong state.
This was fixed in AAudioThread and in AudioStream.

Add "_l" suffix to functions that need to be locked.

Simplify the way that reference counted objects are passed to threads
using incStrong() and decStrong().

Bug: 171296283
Test: atest AAudioTestCases
Change-Id: I034049c4cb9021c6073fff441e49214ee898b804