Enable nullness checking for the IMA extension

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

library/core/src/main/java/com/google/android/exoplayer2/source/ads/AdsMediaSource.java

@@ -271,7 +271,7 @@ public final class AdsMediaSource extends CompositeMediaSource<MediaPeriodId> {
   }
   }
 
 
   @Override
   @Override
-  protected @Nullable MediaPeriodId getMediaPeriodIdForChildMediaPeriodId(
+  protected MediaPeriodId getMediaPeriodIdForChildMediaPeriodId(
       MediaPeriodId childId, MediaPeriodId mediaPeriodId) {
       MediaPeriodId childId, MediaPeriodId mediaPeriodId) {
     // The child id for the content period is just DUMMY_CONTENT_MEDIA_PERIOD_ID. That's why we need
     // The child id for the content period is just DUMMY_CONTENT_MEDIA_PERIOD_ID. That's why we need
     // to forward the reported mediaPeriodId in this case.
     // to forward the reported mediaPeriodId in this case.