audio: Add 'join' method to StreamWorker

This is intended for use in tests where the worker
just executes some actions and then exits by itself.
Use of 'join' instead of 'stop' ensures that the
worker goes through all actions.

Bug: 205884982
Test: atest libaudioaidlcommon_test
Change-Id: I8a9f4f0bb786ee606e3b63a9847f414119716a7d