SDK / exoplayer

ExoPlayer is unable to detect the presence of subtitle tracks in some
MPEG-TS files that don't fully declare them. It's possible for a
developer to provide the list instead, but doing so is quite awkward
without this helper method. This is consistent for how
`DefaultExtractorsFactory` allows other aspects of the delegate
`Extractor` implementations to be customised.

* Issue: google/ExoPlayer#10175
* Issue: google/ExoPlayer#10505

#minor-release

PiperOrigin-RevId: 490214619
(cherry picked from commit ff48faec)