SDK / exoplayer

- Generalize rendererEnabledFlags to be selected track indices through
  ExoPlayerImpl/ExoPlayerImplInternal.
- Selecting an out-of-bound track index (e.g. -1) is equivalent to
  disabling a renderer prior to the generalization.
- A prepared TrackRenderer that exposes 0 tracks is equivalent to a
  TrackRenderer in the STATE_IGNORE state prior to the generalization.

Issue #514.