Add thread-safe information & annotations to necessary fields.

This change is a small incremental step towards better thread-safety.

PiperOrigin-RevId: 598852158

Author: Samrobbo

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

@@ -97,7 +97,9 @@
   private boolean decodeVideo;
   private int sequenceLoopCount;
   private int processedInputsSize;
-  private boolean released;
+
+  // Accessed when switching asset loader.
+  private volatile boolean released;
 
   private volatile long currentAssetDurationUs;
   private volatile long maxSequenceDurationUs;

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

@@ -129,16 +129,34 @@ void onError(
   private final AssetLoaderInputTracker assetLoaderInputTracker;
 
   private final List<SampleExporter> sampleExporters;
-  private final Object setMaxSequenceDurationUsLock;
   private final MuxerWrapper muxerWrapper;
   private final ConditionVariable transformerConditionVariable;
+  private final Object setMaxSequenceDurationUsLock;
 
   private boolean isDrainingExporters;
+
+  @GuardedBy("setMaxSequenceDurationUsLock")
   private long currentMaxSequenceDurationUs;
+
+  @GuardedBy("setMaxSequenceDurationUsLock")
   private int nonLoopingSequencesWithNonFinalDuration;
-  private @Transformer.ProgressState int progressState;
+
   private @MonotonicNonNull RuntimeException cancelException;
 
+  /**
+   * The current {@link Transformer.ProgressState}.
+   *
+   * <p>Modified on the internal thread. Accessed on the application thread (in {@link
+   * #getProgress}).
+   */
+  private volatile @Transformer.ProgressState int progressState;
+
+  /**
+   * The boolean tracking if this component has been released.
+   *
+   * <p>Modified on the internal thread. Accessed on the application thread (in {@link #getProgress}
+   * and {@link #cancel()}).
+   */
   private volatile boolean released;
 
   public TransformerInternal(
@@ -197,8 +215,8 @@ public TransformerInternal(
     }
     compositionHasLoopingSequence =
         nonLoopingSequencesWithNonFinalDuration != composition.sequences.size();
-    sampleExporters = new ArrayList<>();
     setMaxSequenceDurationUsLock = new Object();
+    sampleExporters = new ArrayList<>();
     transformerConditionVariable = new ConditionVariable();
     // It's safe to use "this" because we don't send a message before exiting the constructor.
     @SuppressWarnings("nullness:methodref.receiver.bound")