Implement missing broadcastCustomCommand to legacy controller

And also mention a few restrictions regarding legacy components
in Javadoc.

Issue: androidx/media#293
PiperOrigin-RevId: 522005562
(cherry picked from commit 502969a4)

libraries/session/src/main/java/androidx/media3/session/MediaSession.java

@@ -701,6 +701,9 @@ public class MediaSession {
    * </tr>
    * </table>
    *
+   * <p>Interoperability: This call has no effect when called for a {@linkplain
+   * ControllerInfo#LEGACY_CONTROLLER_VERSION legacy controller}.
+   *
    * @param controller The controller to specify layout.
    * @param layout The ordered list of {@link CommandButton}.
    */
@@ -793,6 +796,9 @@ public class MediaSession {
    *
    * <p>This is a synchronous call and doesn't wait for results from the controller.
    *
+   * <p>Interoperability: This call has no effect when called for a {@linkplain
+   * ControllerInfo#LEGACY_CONTROLLER_VERSION legacy controller}.
+   *
    * @param controller The controller to send the extras to.
    * @param sessionExtras The session extras.
    */
@@ -816,6 +822,9 @@ public class MediaSession {
    *
    * <p>A command is not accepted if it is not a custom command.
    *
+   * <p>Interoperability: This call has no effect when called for a {@linkplain
+   * ControllerInfo#LEGACY_CONTROLLER_VERSION legacy controller}.
+   *
    * @param controller The controller to send the custom command to.
    * @param command A custom command.
    * @param args A {@link Bundle} for additional arguments. May be empty.

libraries/session/src/main/java/androidx/media3/session/MediaSessionLegacyStub.java

@@ -988,6 +988,11 @@ import org.checkerframework.checker.nullness.compatqual.NullableType;
     }
 
     @Override
+    public void sendCustomCommand(int seq, SessionCommand command, Bundle args) {
+      sessionImpl.getSessionCompat().sendSessionEvent(command.customAction, args);
+    }
+
+    @Override
     public void onPlayWhenReadyChanged(
         int seq, boolean playWhenReady, @Player.PlaybackSuppressionReason int reason)
         throws RemoteException {

libraries/session/src/main/java/androidx/media3/session/SessionCommand.java

@@ -123,6 +123,10 @@ public final class SessionCommand implements Bundleable {
 
   /**
    * The extra bundle of a custom command. It will be {@link Bundle#EMPTY} for a predefined command.
+   *
+   * <p>Interoperability: This value is not used when the command is sent to a legacy {@link
+   * android.support.v4.media.session.MediaSessionCompat} or {@link
+   * android.support.v4.media.session.MediaControllerCompat}.
    */
   public final Bundle customExtras;
 
@@ -143,7 +147,9 @@ public final class SessionCommand implements Bundleable {
    * Creates a custom command.
    *
    * @param action The action of this custom command.
-   * @param extras An extra bundle for this custom command.
+   * @param extras An extra bundle for this custom command. This value is not used when the command
+   *     is sent to a legacy {@link android.support.v4.media.session.MediaSessionCompat} or {@link
+   *     android.support.v4.media.session.MediaControllerCompat}.
    */
   public SessionCommand(String action, Bundle extras) {
     commandCode = COMMAND_CODE_CUSTOM;

libraries/test_session_current/src/androidTest/java/androidx/media3/session/MediaControllerCompatCallbackWithMediaSessionTest.java

@@ -981,6 +981,35 @@ public class MediaControllerCompatCallbackWithMediaSessionTest {
   }
 
   @Test
+  public void broadcastCustomCommand_cnSessionEventCalled() throws Exception {
+    Bundle commandCallExtras = new Bundle();
+    commandCallExtras.putString("key-0", "value-0");
+    // Specify session command extras to see that they are NOT used.
+    Bundle sessionCommandExtras = new Bundle();
+    sessionCommandExtras.putString("key-0", "value-1");
+    SessionCommand sessionCommand = new SessionCommand("custom_action", sessionCommandExtras);
+    CountDownLatch latch = new CountDownLatch(1);
+    AtomicReference<String> receivedCommand = new AtomicReference<>();
+    AtomicReference<Bundle> receivedCommandExtras = new AtomicReference<>();
+    MediaControllerCompat.Callback callback =
+        new MediaControllerCompat.Callback() {
+          @Override
+          public void onSessionEvent(String event, Bundle extras) {
+            receivedCommand.set(event);
+            receivedCommandExtras.set(extras);
+            latch.countDown();
+          }
+        };
+    controllerCompat.registerCallback(callback, handler);
+
+    session.broadcastCustomCommand(sessionCommand, commandCallExtras);
+
+    assertThat(latch.await(TIMEOUT_MS, MILLISECONDS)).isTrue();
+    assertThat(receivedCommand.get()).isEqualTo("custom_action");
+    assertThat(TestUtils.equals(receivedCommandExtras.get(), commandCallExtras)).isTrue();
+  }
+
+  @Test
   public void onMediaItemTransition_updatesLegacyMetadataAndPlaybackState_correctModelConversion()
       throws Exception {
     int testItemIndex = 3;