Fixed AudioFlinger not always pausing tracks

If the pause request is received before the AudioTrack buffer was
completelly filled and the track ready for mixing, the pause is
not executed: the track just underruns and stays in pausing state.

The fix consists in considering the track ready for mixing immediately
if pausing.

Change-Id: Ia6cb4703fee2126e41011a6400ea8eeb3a3e5456