SDK / exoplayer

Notes:
- The VP9 one is failing claiming that the util package
  doesn't exist and that LibraryLoader cannot be found.
  Unsure why, since it appears to be setup exactly like
  other extensions (e.g. Opus) that does work.
- @link across modules will not work when generating
  Javadoc for a single module. This is WAI. I subsequent
  change will add an aggregated Javadoc generator that
  will generate Javadoc for all modules together and
  apply cross module @link correctly.

Issue: #2139

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=150864273