NuPlayer: Implement pre-roll for poor network conditions

In poor network conditions, NuPlayer attempts to play out the
available cache. This leads to intermittent and jerky video
and an overall poor playback experience.

Implement pre-rolling (AwesomePlayer way) by setting low
and high buffering thresholds for available cache and
pause/resume player based on the cache duration available.
Also notify the MediaPlayer listeners about buffering
start/end.

CRs-Fixed: 761496
Change-Id: If3b1bad38725b7e2ebb0f195b8a04d3f423f3a1e