Make adding views specific to a ViewGroup's overlay

Adding views to views (possible with the new Overlay API) is weird.
This change moves the view-management facilities of Overlay to a subclass
that is specific to the overlay returned from ViewGroup.getOverlay().
So now you can add drawables to all view overlays, but only add/remove
views to/from the overlay returned from ViewGroup.getOverlay().

Also, the previous approach of using an interface for Overlay was
changed to classes for both ViewOverlay and ViewGroupOverlay.

Finally, this change makes not handling touch correctly the proper,
and documented, behavior of overlay views. There are various tricky issues
to sort out with input in overlays (including click handling as well as focus)
and we don't want developers starting to use overlays as some kind of general
container hierarchy, so we're purposely constraining overlays to have visual-only
behavior.

Issue #8459085 Overlay needs to handle touch correctly

Change-Id: I207b8dbf528f87c92369d270d8b0a6556826d207