SDK / exoplayer

- Allow extension and overriding of renderer creation.
  Several developers have asked for this, so that they
  can use their own renderers (typically extensions to
  the core ones) without losing the ability to use
  SimpleExoPlayer.
- Add option to not attempt extension renderer creation,
  for efficiency.
- Align build variants for internal and external demo
  apps. This is slightly unfortunate, but convergence
  seems necessary for useExtensionRenderers.
- Fix DASH playback tests to use the debug video
  renderer.

Issue #2102

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=140140915