SDK / exoplayer

- Applications should generally use DefaultMediaSourceFactory,
  or their own custom implementation if they need one. Having the
  media2 extension implement its own version directly doesn't seem
  that useful.
- Remove support for CallbackMediaItem. This type of MediaItem
  doesn't go cross-process, and it seems like there would never
  be a use case where an app would need to generate one locally.
  If an app needs to provide data from a custom source, it should
  hook into ExoPlayer's way of doing this (i.e., use a UriMediaItem
  with a custom scheme, and inject a custom DataSource that can
  handle this scheme).

PiperOrigin-RevId: 326914465