SDK / exoplayer

- Removed the PES_STRIPPED flag. It's unnecessary. We can strip
  PES in the TS extractor instead.
- Made nearly all of the object classes in DvbParser immutable.
  Else it's non-obvious that none of this state can be mutated.
- Made a a lot of the methods in DvbParser static for the same
  reason.
- Removed unnecessary null checks, code that was never executed,
  unused fields etc.
- Add proper flushing of DvbParser, to prevent corrupt output
  following a seek.