Allow to ignore/skip items in a playlist

I use ConcatenatingMediaSource to play a track list in ExoPlayer. Some of the media sources are downloaded files. At any point during playback, user can go into Offline mode where only downloaded tracks should be played.

Is it possible to somehow flag these downloaded media sources and make Exo skip non-flagged tracks? Or is the only solution to use concatenatingMediaSource.removeSource() and manually remove all non-downloaded sources?

I've been looking into TrackSelector class, but couldn't find anything helpful there.