audio: Fix handling of quick worker completion in StreamWorker

In tests, it is possible that the worker exits quickly,
prior to the moment when the thread controller begins
waiting for it to start. Improper handling of this case
was causing intermittent lock-ups of VTS tests.

Bug: 205884982
Test: atest libaudioaidlcommon_test
Change-Id: I13a83113b32b859e212f1a517ad61bf5b8d43365