SDK / exoplayer

This is a proof of concept for cleanup we should do for all of
our DataSource implementations as we move toward stabilizing
parts of the API.

- Move all XDataSourceFactory classes to be inner classes.
- Remove chained constructors for XDataSourceFactory classes. Keep
  required args going through constructors. Use setters for the
  rest.
- Not applicable in this case, but we probably want to deprecate
  all but the no-arg method for instantiating eac XDataSource
  instance (with the all-arg method kept but with the intention
  of making it package private).

PiperOrigin-RevId: 274162076