Correct the time returned by AudioPlayer::getMediaTime

The change 80a68dee was added to
eliminate returning negative media positions.

In the change all negative values of realTimeOffset were discarded.
The value realTimeOffset accounts for the latency and the
number of samples played since last feeding the AudioTrack and is
generally negative. By discarding this value for negative values the
time returned does not account for latency or the number of samples
played.

Change-Id: I470450934228732e34539c227365a225be74ccc8