SDK / exoplayer

Fix some EventListener logic in ExoPlayerImpl

We currently have multiple places in ExoPlayerImpl that assign PlaybackInfo
instances and then inform listeners of all current changes. This is not ideal
because it causes multiple issues:
 1. Some changes may easily be forgotten, e.g. there are clearly some checks
    missing to see if isPlaying changed (e.g. in seekTo or setMediaSources)
 2. Some callbacks didn't check if the value actually changed before sending
    the callback (e.g. for the timeline change in setMediaSources - if the
    timeline is still the same, we shouldn't send a onTimelineChanged event).
 3. Having multiple callbacks in a single Runnable changes the order of
    listener invocations slightly: Currently all events for one listener will
    be send first before moving to the next listener. It should however send
    a single event to all listeners first before moving to the next event.

All these issues can be solved by always using updatePlaybackInfo and never
assigning playbackInfo directly in another place.

Some tests needed to be updated as well because of issues (2) and (3). Also
added a new test to cover issue (1).

PiperOrigin-RevId: 302844981