SDK / exoplayer

Improve PlaybackStatsListener by using onEvents callback

Previously the PlaybackStatsListener needed to handle all events
individually, which required to keep some state of the player and
to resolve potentially transient state changes.

Using onEvents allows to channel all simultanous updates through
one method so that no transient player state and other
inconsistencies need to be handled. This makes the logic easier
to read.

In addition it also allows to resolve all simultaneous events to
use one EventTime (with one timestamp).

#exofixit

PiperOrigin-RevId: 344415459