SDK / exoplayer

- Move nearly all logic onto the calling thread (i.e. the thread
  calling open/read/close), to make threading correctness more
  obvious.
- Document which variables are read/written from which thread, and
  why the call sequences are safe.
- Fix thread safety issue that I think could probably cause data
  corruption in the case of a read timeout followed by another
  request into the DataSource.

Also:

- Relaxed content length checking to be consistent with the other
  http DataSource implementations, and avoided parsing the headers
  where they're not used.
- Fixed missing generics in CronetDataSourceFactory.
- Added TODO to work with servers that don't support partial range
  requests.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=135702217