audio: handle audio underruns using silence playback

Audio underruns can be handled either by stopping pcm device and
preparing it again or by playing silence frames till application
fills the buffer again. On average, stopping and preparing pcm device
takes 2-4 times more time than just playing silence frames and let
the application catch-up. So switching to that approach now.

Bug: 20226809
Bug: 20300203
Change-Id: I53a1d32879e594d15fa2b0142852649edb2b5835
2 files changed