SDK / exoplayer

With this CL:
* The first supported video track found is selected.
* The first supported audio track with preferred language is selected.
  If none, we fallback to the first supported one.
* For text, we only present a selection if one of the tracks has the
  preferred language and the track is flagged as forced.

TODO list:
* Add a selection policy for video (probably related with adaptiveness).
* We should decide what to do with the default flag.
* Perhaps, if no audio with the preferred language(assuming there is one)
  is found, we should fall back to a text track that has the preferred
  language.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=122985006