audio: Prevent priority inversions in aidl StreamWorker

Avoid taking a lock in high priority worker threads
without a real need.

Bug: 205884982
Test: atest libaudioaidlcommon_test --iterations
Merged-In: I8cc0f5cb58752b7b7d413a9f4e46093c39445892
Change-Id: I8cc0f5cb58752b7b7d413a9f4e46093c39445892
(cherry picked from commit d989a4b6)