Document ExoMediaDrm listener methods as optional

Also document FrameworkMediaDrm overrides as dependent on API level.

These two methods call through to equivalent methods on MediaDrm that
were added in 23:
https://developer.android.com/reference/android/media/MediaDrm#setOnExpirationUpdateListener(java.util.concurrent.Executor,%20android.media.MediaDrm.OnExpirationUpdateListener)

PiperOrigin-RevId: 303102370

library/core/src/main/java/com/google/android/exoplayer2/drm/ExoMediaDrm.java

@@ -224,13 +224,40 @@ public interface ExoMediaDrm {
 
   }
 
-  /** @see MediaDrm#setOnEventListener(MediaDrm.OnEventListener) */
+  /**
+   * Sets the listener for DRM events.
+   *
+   * <p>This is an optional method, and some implementations may only support it on certain Android
+   * API levels.
+   *
+   * @param listener The listener to receive events, or {@code null} to stop receiving events.
+   * @throws UnsupportedOperationException if the implementation doesn't support this method.
+   * @see MediaDrm#setOnEventListener(MediaDrm.OnEventListener)
+   */
   void setOnEventListener(@Nullable OnEventListener listener);
 
-  /** @see MediaDrm#setOnKeyStatusChangeListener(MediaDrm.OnKeyStatusChangeListener, Handler) */
+  /**
+   * Sets the listener for key status change events.
+   *
+   * <p>This is an optional method, and some implementations may only support it on certain Android
+   * API levels.
+   *
+   * @param listener The listener to receive events, or {@code null} to stop receiving events.
+   * @throws UnsupportedOperationException if the implementation doesn't support this method.
+   * @see MediaDrm#setOnKeyStatusChangeListener(MediaDrm.OnKeyStatusChangeListener, Handler)
+   */
   void setOnKeyStatusChangeListener(@Nullable OnKeyStatusChangeListener listener);
 
-  /** @see MediaDrm#setOnExpirationUpdateListener(MediaDrm.OnExpirationUpdateListener, Handler) */
+  /**
+   * Sets the listener for session expiration events.
+   *
+   * <p>This is an optional method, and some implementations may only support it on certain Android
+   * API levels.
+   *
+   * @param listener The listener to receive events, or {@code null} to stop receiving events.
+   * @throws UnsupportedOperationException if the implementation doesn't support this method.
+   * @see MediaDrm#setOnExpirationUpdateListener(MediaDrm.OnExpirationUpdateListener, Handler)
+   */
   void setOnExpirationUpdateListener(@Nullable OnExpirationUpdateListener listener);
 
   /** @see MediaDrm#openSession() */

library/core/src/main/java/com/google/android/exoplayer2/drm/FrameworkMediaDrm.java

@@ -114,7 +114,14 @@ public final class FrameworkMediaDrm implements ExoMediaDrm {
                 listener.onEvent(FrameworkMediaDrm.this, sessionId, event, extra, data));
   }
 
+  /**
+   * {@inheritDoc}
+   *
+   * @param listener The listener to receive events, or {@code null} to stop receiving events.
+   * @throws UnsupportedOperationException on API levels lower than 23.
+   */
   @Override
+  @RequiresApi(23)
   public void setOnKeyStatusChangeListener(
       @Nullable ExoMediaDrm.OnKeyStatusChangeListener listener) {
     if (Util.SDK_INT < 23) {
@@ -135,7 +142,14 @@ public final class FrameworkMediaDrm implements ExoMediaDrm {
         /* handler= */ null);
   }
 
+  /**
+   * {@inheritDoc}
+   *
+   * @param listener The listener to receive events, or {@code null} to stop receiving events.
+   * @throws UnsupportedOperationException on API levels lower than 23.
+   */
   @Override
+  @RequiresApi(23)
   public void setOnExpirationUpdateListener(@Nullable OnExpirationUpdateListener listener) {
     if (Util.SDK_INT < 23) {
       throw new UnsupportedOperationException();