libktpcall: Simplify the controllers API

* Consider all source & sink controllers private classes by themselves
and get rid of the *ControllerPrivate classes.
* Move the sink managers into a separate source file.
* Fix a few things in BaseSinkManager.
* Move some parts of the implementation from Audio/VideoSinkManager
to the Audio/VideoSinkController classes.
* Add subclasses of CallContentHandler that expose the various pipeline
controls that used to be exposed by the source/sink controllers.