onGetChildren
open fun onGetChildren(session: PillarboxMediaLibrarySession, browser: MediaSession.ControllerInfo, parentId: String, @IntRange(from = 0) page: Int, @IntRange(from = 1) pageSize: Int, params: MediaLibraryService.LibraryParams?): ListenableFuture<LibraryResult<ImmutableList<MediaItem>>>
Called when a PillarboxMediaBrowser requests the child media items of the given parent id.