Check whether a session is still managed before removing

When the controller of the `MediaNotificationManager` is disconnected,
the session is removed from the service without checking whether the
session hasn't already been removed. This caused flakiness in `MediaSessionServiceTest.addSession()`.

Because there is a public API `MediaSessionService.removeSession()`,
the controller can't make an assumption whether the session is still
contained in the service when being disconnected.

#minor-release

PiperOrigin-RevId: 572568350

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

@@ -392,7 +392,9 @@ public void onAvailableSessionCommandsChanged(
 
     @Override
     public void onDisconnected(MediaController controller) {
-      mediaSessionService.removeSession(session);
+      if (mediaSessionService.isSessionAdded(session)) {
+        mediaSessionService.removeSession(session);
+      }
       // We may need to hide the notification.
       mediaSessionService.onUpdateNotificationInternal(
           session, /* startInForegroundWhenPaused= */ false);