SDK / exoplayer

Moved the behaviors related to Cue's to the WebvttCueParser class.
This way, the parsing methods will be more easily accessible to
other classes, such as the MP4Webvtt parser. This class also has
some methods that require state to avoid repetitive avoidable
allocations. The method visibility is subject to changes in
further CLs.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=111616824