Disable the muxer watchdog if all tracks have ended.

PiperOrigin-RevId: 484512661
(cherry picked from commit 2390322a)

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

@@ -195,6 +195,9 @@ import org.checkerframework.checker.nullness.qual.MonotonicNonNull;
    */
   public void endTrack(@C.TrackType int trackType) {
     trackTypeToIndex.delete(trackType);
+    if (trackTypeToIndex.size() == 0) {
+      abortScheduledExecutorService.shutdownNow();
+    }
   }
 
   /**