SDK / exoplayer

- Made parts of DownloadActions public that were previously hidden.
  This is necessary so that when an app is listening to the
  DownloadManager, it can tell something about the tasks.
- Simplified DownloadActivity some more. The helpers may migrate
  into their corresponding modules at some point, and also be
  converted to return structured TrackGroupArrays rather than a
  flat list of Formats.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=195269824