MPD file may include multiple EventStreams in its Periods, which contains Events
that the application may need to handle/respond to.
This change adds support for parsing the EventStream/Event nodes from MPD
file, and exposing these EventStreams as a metadata sample stream that application
can respond in a similar way to other metadata events.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=175017697
