audio: Allow stopping a StreamWorker from the looping thread

Enhance the return type of the 'workerCycle' to allow it
exiting without inducing an error on the controller side.

Also, put StreamWorker into a namespace.

Bug: 205884982
Test: atest libaudioaidlcommon_test --iterations
Change-Id: I3b27028b10f80f27985040cae8f8b0e6ab63ddad