Correct the time returned by AudioPlayer::getMediaTime

The change 80a68deec52c9a1b47215ed68526206ec88197f8 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
stagefright: Fix seeking in MPEG4 container - do not merge

- seek based on user-visible timestamp
- seek to previous, next and closest exactly
- also update OggExtractor seek to same logic

Bug: 12786906
Change-Id: I2d5e1fbff6469b6f081a284be2222a4cb50aa754