Fix for RootElement Sections list

The RootElement class has a List of Sections. In MT.D this is initialised by [default](https://github.com/migueldeicaza/MonoTouch.Dialog#L2465). However in CrossUI.Dialog version of MT.D it is just a [getter/setter](https://github.com/slodge/MvvmCross/blob/vnext/CrossUI/CrossUI.Touch/Dialog/Elements/RootElement.cs#L117).

I've updated the constructors to make sure the list is initialised.