SDK / exoplayer

This paves the way for SegmentTemplate and SegmentList based
mpds, which will implement DashSegmentIndex directly rather than
parsing an index from the media stream.

- Define DashSegmentIndex.
- Make use of DashSegmentIndex in chunk sources.
- Define an implementation of DashSegmentIndex that wraps a SegmentIndex.
- Add method that will allow Representations to return a DashSegmentIndex
  directly in the future.
- Add support for non-contiguous index and initialization data in media streams.
  For the Webm case this isn't enabled yet due to extractor limitations.
- Removed ability to fetch multiple chunks. This functionality does not extend
  properly to SegmentList and SegmentTemplate variants of DASH.