SDK / exoplayer

The extended onPositionDiscontinuity callback can be used to improve some
listener classes:
 - Listening to onTimelineChanged to detect discontinuities is no longer needed.
 - Listening to onSeekStarted is no longer needed as the start position is part
   of the onPositionDiscontinuty callback.
 - The exact old position is also useful for media time history logging.

As a side effect, removing onSeekStarted handling from PlaybackStatsListener
fixes Issue: #8675 that was caused by the special EventTime handling for
onSeekStarted.

PiperOrigin-RevId: 365558959