Intermittent pop when starting or stopping playback

[Boris Burtin]

I've noticed intermittent pops in the audio output when starting or stopping playback. Doing a quick fade in/out helped, but the problem still occasionally happens. I noticed the following lines in the device log:

05-14 15:58:40.753 D/AudioHardware( 90): AudioHardware pcm playback is exiting standby. 05-14 15:58:48.780 D/AudioHardware( 90): AudioHardware pcm playback is going to standby.

My current theory is that the pops occur when the audio device goes in and out of stand by. It may be possible to avoid the problem if we don't release the AudioTrack, but instead keep it in a pool and reuse it as long as the app is in the foreground.