Make input media duration volatile

The duration in TransformerInternal.ComponentListener is set on the
Transformer internal thread, and is read on the playback thread. Making
this field volatile ensures that the playback thread reads the updated
value.

PiperOrigin-RevId: 493908385

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

@@ -377,9 +377,9 @@ import org.checkerframework.checker.nullness.qual.MonotonicNonNull;
     private final MediaItem mediaItem;
     private final FallbackListener fallbackListener;
 
-    private long durationUs;
     private int tracksAddedCount;
 
+    private volatile long durationUs;
     private volatile boolean trackRegistered;
 
     public ComponentListener(MediaItem mediaItem, FallbackListener fallbackListener) {