Keep context menu above other popups, use dismiss()

Keeps the context menu above other popups by setting its Z value to
the highest Z value among the children of Overlay.overlay, plus 1.

Also using dismiss() instead of close() because dismiss will
completely close a menu. Doesn't really matter here since
there are no submenus, but it's the correct thing to do.

BUG: 428631
FIXED-IN: 5.82