selectSecondarySubtitle

Shows a second subtitle track where SubtitleConfig.secondaryPlacement puts it, at the top of the picture by default, or clears it with null. SubtitleConfig.secondaryLanguages can choose one at each open instead.

The secondary track's text leaves where its author put it for that place, so the two tracks never sit on each other; the primary stays where its author put it. External tracks (negative ids) are allowed on either slot. Selecting the track that already fills the other slot throws IllegalArgumentException. Tracks.selectedSecondarySubtitle reports the selection.