Fix threading issueis in SequenceAssetLoaderListener

Callbacks onTrackCount and onTrackAdded can be called simultaneously
from different threads.

Before this fix, it was possible for the MuxerWrapper and
FallbackListener track count to never be set, or to be set
with incorrect values.

PiperOrigin-RevId: 514779719

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

@@ -440,8 +440,8 @@ import org.checkerframework.checker.nullness.qual.MonotonicNonNull;
                 ERROR_CODE_FAILED_RUNTIME_CHECK));
         return;
       }
-      trackCountsToReport.decrementAndGet();
       tracksToAdd.addAndGet(trackCount);
+      trackCountsToReport.decrementAndGet();
     }
 
     @Override
@@ -465,12 +465,11 @@ import org.checkerframework.checker.nullness.qual.MonotonicNonNull;
       } else {
         outputHasVideo.set(true);
       }
-      if (trackCountsToReport.get() == 0 && tracksToAdd.get() == 1) {
+      if (trackCountsToReport.get() == 0 && tracksToAdd.decrementAndGet() == 0) {
         int outputTrackCount = (outputHasAudio.get() ? 1 : 0) + (outputHasVideo.get() ? 1 : 0);
         muxerWrapper.setTrackCount(outputTrackCount);
         fallbackListener.setTrackCount(outputTrackCount);
       }
-      tracksToAdd.decrementAndGet();
 
       return trackInfo.shouldTranscode;
     }