SDK / exoplayer

It's only "needed" for HLS, and this requirement will
go away soon. It's safe to remove the flag ahead of
this because a TrackSelector can/should not attempt to
adapt between multiple audio/text tracks.

Also remove unnecessary restrictions on TrackGroups
being non-empty and only of known types.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=128376601