SDK / exoplayer

- Remove unused method in DashChunkSource.
- Remove inputEncoding parameter for subtitle parsers. We're
  ignoring it in all but one of the parsers, and for the one
  that does use it, it'll only ever receive null, since that's
  all we're passing.
- Make TextTrackRenderer advance to the next subtitle even if
  the current one hasn't finished, in the case that they overlap.
  This shouldn't ever really happen, but it seems best to trust
  the start time of the new sample rather than the last event
  time of the previous one.