SDK / exoplayer

This CL allows passing multiple formats describing CC channels to the
TS payload reader factory. As a simple usecase, ATSC can expose both
608 channels by passing a two element list with the corresponding
accessibility channels. The HLS media source can construct this list
from the EXT-X-MEDIA:TYPE="CLOSED-CAPTIONS" tags, including language
and selection flags. The interface extends without modification to
708.

Pending work:
* Multiple CC channels in HLS.
* caption_service_descriptor parsing for overriding the user's selection.
* 708 support in SEI reader.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=148030293