The looping media source doesn't convert the media period id to the externally
visible media period id. And the merging media source reports media period
creations multiple times which will break listeners assuming a media period
with a specific id will only be created once.
Also amend the doc for MediaSource.createPeriod to reflect that media periods
created in parallel do not actually have the same id.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=206327241
656c2172dd