DownloadHelper is in the ExoPlayer module, so there's no reason
why it can't use ExoPlayer specific track selections. That said,
we want our UI components to operate on generic
TrackSelectionParameters, and we want such UI components to be
useful for selecting tracks for download. To keep this interop,
it's necessary to have DownloadHelper accept generic
TrackSelectionParameters, or to require application code to
convert them. The first approach seems preferable!
PiperOrigin-RevId: 432158846
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