auhal: implement pause by output silence instead of stopping output

Stopping and restarting the audio unit or device might take a
considerable amount of time. This leads to flushes, and
the user notice a delay for resuming audio.

Auhal does not have a better API for pause, so lets only play silence
for now. This fixes resume delays with analog mode, and might improve
delays for digital mode (through they are still occuring from
time to time here).

close #9536