SDK / exoplayer

This method needs to be called whenever the track selection of the current
loading period changes, but also when the loading period itself (and thus
the "loading track selection") changes. These are the same situations in which
we update the loading media period id and thus we can move both updates in
a common helper method.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=213959982