MediaPlayerService: fix AudioSink latency

The AudioSink latency is currently cached when the associated AudioTrack
is created. However, the AudioTrack latency can change if the AudioTrack is moved
from one output stream to another.
The AudioPlayer must also periodically update its view of the latency
as it is needed to compensate the real audio time used for A/V sync.

This fixes an A/V sync problem seen when switching A2DP on and off while
playing a video.

Change-Id: I28b24049ca114e1af3e24791dcc900f463536ba4