SDK / exoplayer

- Migrate demo app to use new APIs.
- Add multi-track support for ExtractorSampleSource case.
- Add multi-track support for SmoothStreaming use case.

The final step is to add support back for the DASH use case and
delete MultiTrackChunkSource. This is blocked on multi-period support
landing, in order to prevent a horrendous merge conflict. We also
need to update HLS to expose sensible track information.

Issue: #514