SDK / exoplayer

When we have multiple overrides for TrackGroups associated with
one renderer, we need to look at all of them to find the non-empty
one. Empty ones should only be used to remove previously selected
tracks for this group and otherwise be ignored.

Currently this is broken because the first override (no matter if
it's empty or not) is used as the final selection for this renderer.

Issue: google/ExoPlayer#9649

PiperOrigin-RevId: 407792330