SDK / exoplayer

adPlaybackState is now non-null, and the uninitialized case is covered by a new
boolean hasAdPlaybackState. Position progress updates are now non-null and
initialized with IMA's VIDEO_TIME_NOT_READY constant.

Also fix some misc code issues:
- Remove empty branch for SmoothStreaming (Android Studio warns about this).
- Tidy onTimelineChanged and onPositionDiscontinuity and the methods they call
  to improve naming.
- Remove logging for IMA events after release, as these methods are expected to
  be called in the current IMA SDK behavior.

PiperOrigin-RevId: 308977116