SDK / exoplayer

- This removes the need for Variant and Rendition to have a common
  base class, allowing the url field to be marked as @Nullable in
  Rendition but not in Variant.
- The addition of mediaPlaylistUrls is needed for the new StreamKey
  indexing for HLS. It's also convenient in a couple of places (e.g.
  HlsDownloader), where a list of all media playlist URLs is needed.
- Lots of places where HlsUrl was passed only needed the actual
  URL (not the Format, which is the other piece of HlsUrl). Passing
  just the URL is a little simpler, and resolves some of the naming
  confusion.

Issue: #5596
Issue: #2600
PiperOrigin-RevId: 240970466