SDK / exoplayer

- Update relevant documentation
- Use individual modules for the demo app. Given it's
  preferable to use them over the full library, and that
  many people probably use the demo app as a starting
  point, we should set the right example even though we
  currently need to include all of them.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=153454898