This is needed for several use cases:
- ExtractorSampleSource with option to play both embedded and out-of-band
subtitles.
- HLS multi-audio and out-of-band-webvtt.
- 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.
