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
