HLS: More control over buffering + tweak caption impl.

- Move all three buffering constants to a single class (the
  chunk source).
- Increase the target buffer to 40s for increased robustness
  against temporary network blips.
- Make values configurable via the chunk source constructor.
- Treat captions as a text track for HLS. This allows them to
  be enabled/disabled through the demo app UI.

Issue: #165