SDK / exoplayer

- Remove TsExtractor's knowledge of Sample.
- Push handling of Sample objects into SampleQueue as much
  as possible. This is a precursor to replacing Sample objects
  with a different type of backing memory. Ideally, the
  individual readers shouldn't know how the sample data is
  stored. This is true after this CL, with the except of the
  TODO in H264Reader.
- Avoid double-scanning every H264 sample for NAL units, by
  moving the scan for SEI units from SeiReader into H264Reader.

Issue: #278