Fix issue 3217707.

The problem is that when the A2DP headset is disconnected, there is a transition
period during which the A2DP sink pumps data at a very high pace.
This makes that:
1 the audio flinger mixer thread spins and starves binder threads thus delaying
the completion of the A2DP output stream shutdown
2 we read the audio http audio stream faster than normal and we reach the end of stream
for audio while video is still playing if the streamed file is small enough.

The fix consists in detecting abnormal short write intervals and sleep to restore
a normal write pace.

Change-Id: Iab127882494ab0e26266371dc0ce5c2ff6fa476e