It doesn't look like TrackSelectionPolicy is going to be
useful other than with DefaultTrackSelector, and it's kinda
confusing dealing with both "selector" and "policy"
terminology. This change does the following:
DefaultTrackSelector -> selector.MappingTrackSelector
DefaultTrackSelectionPolicy -> selector.DefaultTrackSelector
TrackSelectionPolicy -> [deleted]
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=127196326
