NuPlayer: Allow audio drain to fill buffer when paused

If a pause lasts more than a few seconds, the audio device
enters standby.  When the audio device comes out of standby,
it typically requests a large amount of data to fill its buffers
(especially severe for deep buffer PCM mode).

To avoid a glitch, continue to fill the buffer (until no more data
can be written) when paused.

Also, we do not start the audio sink if we are paused to avoid
audio playback in video preview mode.

Bug: 19062223
Bug: 21198655
Change-Id: Ifb3f181d5e810283381b65f2af16a4c71df0b04f