When the dynamic media source contains multiple empty timelines in a row and some
of them dynamically change to a non-empty timeline, the window and period indices
are not updated correctly because the index of the changed child source is wrong.

To fix this bug, the child index is added to the media period holder to have direct
access on the current child index to prevent ambiguity.

Furthermore, the uid is changed to be the hash code of the MediaSourceHolder not the
MediaSource itself to allow adding the same MediaSource twice without violating the
unique uid policy.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=183973462