SDK / exoplayer

This CL adds the support of CSS styling in Cues through id and "universal" cue selector.
The more sophisticated selectors will be left for later, because they requier a bit more
complex logic. Also narrowed a little bit the responsibilities of the WebvttCueParser to
move some to the WebvttParser.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=119547731