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
