audio: Use 'join' instead of 'stop' for stream workers

Calling 'stop' can cause the worker thread to terminate
before it handles the 'halReservedExit' command. This should
be avoided because a proper exit sequence may do cleanups.
Since all stream workers must handle the 'halReservedExit'
command, use of 'stop' should not be needed (if the thread
code gets stuck on a call to drivers, calling 'stop' will
not interrupt this), thus it is being replaced by 'join'.

Bug: 344482249
Test: atest CtsMediaAudioTestCases
Test: atest VtsHalAudioCoreTargetTest
(cherry picked from https://android-review.googlesource.com/q/commit:657284d4ef31a9829c13fef818fe2d41f8c0382e)
Merged-In: If13f7239423657b80091239ff67e7fe350957e2e
Change-Id: If13f7239423657b80091239ff67e7fe350957e2e