This adds callbacks for creating, releasing, and starting to read from media
periods.
Such events allow listeners to keep a list of active media periods. This is
useful to determine when no further events for a certain media period are
expected. It also allows listeners to associate renderer events unambigiously
with a reading media period.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=190462717
