Add missing final keywords and clarify contract of some methods

Some of the methods are meant to be overridden by apps, but
should never be called directly to avoid unintended side
effects. Other util public util methods should be final to
make it clearer that they shouldn't be overriden.

PiperOrigin-RevId: 780614691

Author: tonihei

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

@@ -251,6 +251,8 @@ public void onCreate() {
    * Return a {@link MediaSession} that the controller will connect to, or {@code null} to reject
    * the connection request.
    *
+   * <p>Note: This method must not be called directly by app code.
+   *
    * <p>The service automatically maintains the returned sessions. In other words, a session
    * returned by this method will be added to the service, and removed from the service when the
    * session is closed. You don't need to manually call {@link #addSession(MediaSession)} nor {@link
@@ -559,7 +561,7 @@ public final void setShowNotificationForIdlePlayer(
    * <p>This method must be called on the main thread.
    */
   @UnstableApi
-  public boolean isPlaybackOngoing() {
+  public final boolean isPlaybackOngoing() {
     return getMediaNotificationManager().isStartedInForeground();
   }
 
@@ -573,7 +575,7 @@ public boolean isPlaybackOngoing() {
    * <p>This method must be called on the main thread.
    */
   @UnstableApi
-  public void pauseAllPlayersAndStopSelf() {
+  public final void pauseAllPlayersAndStopSelf() {
     getMediaNotificationManager().disableUserEngagedTimeout();
     List<MediaSession> sessionList = getSessions();
     for (int i = 0; i < sessionList.size(); i++) {
@@ -648,6 +650,8 @@ public void onUpdateNotification(MediaSession session) {
    * Called when a notification needs to be updated. Override this method to show or cancel your own
    * notifications.
    *
+   * <p>Note: This method must not be called directly by app code.
+   *
    * <p>This method is called whenever the service has detected a change that requires to show,
    * update or cancel a notification with a flag {@code startInForegroundRequired} suggested by the
    * service whether starting in the foreground is required. The method will be called on the