SDK / exoplayer

Fixes github issue #2339 [https://github.com/google/ExoPlayer/issues/2339]
Fixes github issue #2551 [https://github.com/google/ExoPlayer/issues/2551]

* Update the instructions to check out specific versions of libvpx and
  libyuv that are known to work with our build scripts.
* Forcing a particular version of libyuv because recent versions of
  libyuv are dependent on libjpeg (which isn't needed for the purpose
  of this extension).
* Going forward, let's keep generate_libvpx_android_configs.sh in sync
  with whatever version is specifed in the instructions in README.md
  (as of now it is v1.6.1).

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=150208517