Add transition support to PopupWindow

Allows framework-added windows to manually specify surface insets, which
enables us to wrap the popup window's elevated content view with a root
view, which in turn allows us to use the Transition API for popup window
transitions.

Fixes a bug where the root view's render node forced clipping.

Bug: 13211166
Change-Id: I303dfa55a052cdf5d3b1485422529123e3cc867a