Dispatch the actual PopupWindow dismiss callback to PopupMenu

Previously the dismiss callback was called immediately after the menu
received a close request; however, the dismiss callback implies that
the menu's window has finished animating and been removed from the
window manager.

Also cleans up handling of mPopup in MenuPopupHelper to prevent
unnecessary MenuPopup allocations and convert unnecessary fields into
method arguments.

Bug: 25323707
Change-Id: I8e3877ae6c40b4d0f1df23a4ff4fa48a7df34e0d