The WindowManager side of Surface.java holds a SurfaceControl, while the client-side holds a Surface. When the client is in the system process, Surface.java holds both (which is a problem we'll try to fix later).
SurfaceControl is used for controling the geometry of the surface (for the WM), while Surface is used to access the buffers (for SF's clients).
SurfaceFlingerClient now uses the SurfaceID instead of Surface*.
Currently Surface still has the SurfaceControl API and is implemented by calling into SurfaceControl.
To deal with Java's lack of destructors and delayed garbage collection, we used to duplicate Surface.cpp objects in some case; this caused some issues because Surface is supposed to be reference-counted and unique.
