How to create my own callback-method of MediaControllerCompat

Question

I'm developing simple music player which have client-server architecture(there is MediaControllerCompat object in Activity that sends callbacks to MediaSessionCompat in the MusicService). I need to change current playlist, when user choose another folder. So, there is MediaBrowserCompat.SubscriptionCallback object in activity and it has method onChildrenLoaded() when I get new List of songs. In this method I need to send callback to media session to delete music which was in previous playlist(look at code). It's a problem MediaControllerCompat is final class and I can't create a subclass where I would add some method to solve my problem.

override fun onChildrenLoaded(parentId: String, children: List<MediaBrowserCompat.MediaItem>) {
            super.onChildrenLoaded(parentId, children)

            //TODO: check is there some music in current playlist, if there is some music, clear playlist


            //add all media
            for (mediaItem in children) {
                mediaController.addQueueItem(mediaItem.description)
            }

            //call prepare now so pressing play
            mediaController.transportControls.prepare()
        }