Refactor `MuxerWrapper` handling of track details.

This brings together the multiple details about a muxer track, and
reduces the need for additional variables and more complicated track
tracking.

PiperOrigin-RevId: 499872145

library/transformer/src/main/java/com/google/android/exoplayer2/transformer/Muxer.java

@@ -78,10 +78,10 @@ public interface Muxer {
   }
 
   /**
-   * Adds a track with the specified format, and returns its index (to be passed in subsequent calls
-   * to {@link #writeSampleData(int, ByteBuffer, boolean, long)}).
+   * Adds a track with the specified format.
    *
    * @param format The {@link Format} of the track.
+   * @return The index for this track, which should be passed to {@link #writeSampleData}.
    * @throws MuxerException If the muxer encounters a problem while adding the track.
    */
   int addTrack(Format format) throws MuxerException;

library/transformer/src/main/java/com/google/android/exoplayer2/transformer/TransformerInternal.java

@@ -465,7 +465,8 @@ import org.checkerframework.checker.nullness.qual.MonotonicNonNull;
     // MuxerWrapper.Listener implementation.
 
     @Override
-    public void onTrackEnded(@C.TrackType int trackType, int averageBitrate, int sampleCount) {
+    public void onTrackEnded(
+        @C.TrackType int trackType, Format format, int averageBitrate, int sampleCount) {
       if (trackType == C.TRACK_TYPE_AUDIO) {
         transformationResultBuilder.setAverageAudioBitrate(averageBitrate);
       } else if (trackType == C.TRACK_TYPE_VIDEO) {