SDK / exoplayer

- Add a top level failure type (source/renderer/unexpected),
  and convenience methods for retrieving the underlying cause
  without needing to cast.
- Also add renderer index in the case of renderer failures.
- setIndex/getIndex is a little . . . unclean, but alternatives
  involve either having the top line of the stack trace be a
  non-interesting line, or loads of try/catch blocks in
  ExoPlayerImplInternal.

Issue: #777
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=114763073