Fix broken popup menu in Shelf (andromeda).

This addresses a few oversights from an earlier CL:
1. In MenuPopupHelper#show() make sure to create a new MenuPopup in
   case the earlier one was dismissed.
2. Ensure the on-dismiss listener gets called even if the MenuPopupHelper's
   MenuPopup was previously closed and if a new one is opened.
3. Handle global layout changes properly by having the MenuPopup
   re-drawing/positioning itself.

Bug: 23973158
Change-Id: Iee866079770026f0fe17814892abc9825f9760a2