SDK / exoplayer

This has a few benefits:
  * Aligns the Builder constructors with the setters
    (setRenderersFactory is missing, but can be easily added in a
    follow-up change).
  * Allows DefaultMediaSourceFactory to be stripped by R8 and
    makes the shrinking dev guide for the cases of providing a custom
    MediaSourceFactory or directly instantiating MediaSource instances
    less weird too.

#minor-release

PiperOrigin-RevId: 405632981