aaudio: allow stream to restart after RESULT_STOP

We used to return an error if an MMAP callback thread
had not been joined and we tried to create a new thread.
Now we just join the old thread before creatng a new one.
It is safe to join a thread that has already exited.

Bug: 193920793
Test: atest CtsNativeMediaAAudioTestCases
Change-Id: I15e6a6dad09d1ad3e0ce36c261fcf2c9ed46259d