Make ModalViewContainer size dialogs to cover its body

[rjrjr]

Our internal fork of ModalViewContainer sizes / places dialog windows to track the size of the body view they cover. Makes them much more flexible, with no apparent penalty.

[rjrjr]

Most of the work in this is done in an implementation of ViewTreeObserver.OnGlobalLayoutListener. An alternative to baking this behavior into ModalViewContainer would be to put that in its own file as an extension on Dialog, and make sure that ModalViewContainer is open enough that implementors can take advantage.

See also #25