audio: Implement setting name and priority in StreamWorker

Audio threads need to be able to set their priority.
Also, traditionally these worker threads set a custom
thread name. Implement this functionality in StreamWorker.

Since initialization steps can fail, implement simple
error reporting via a string field `mError`. The state
of the string field replaces the dedicated `ERROR` worker
state.

Bug: 205884982
Test: atest libaudioaidlcommon_test --iterations
Change-Id: Ie9ab94922d47f277a4993a90b478a2fa76657923