Pull out widget helpers into their own classes.

This is in service of desktop-style submenus. Follow up CLs will make
use of the newly independent classes. MOST of the changes in this CL
are simply moving code from one place to another. Below are
descriptions of the new files added:

DropDownListView:
Decouple this from ListPopupWindow, so other "menu popup"
implementations [forthcoming] may make use of it.
This class is UNCHANGED except for the addition of the
setListSelectionHidden method.

ForwardingListener:
Decouple this from ListPopupWindow, so that other clients of this
listener can expect a more general ShowableListMenu rather than a
concrete ListPopupWindow.  This will be useful later when we want to
use something other than ListPopupWindow to show a menu.
This class is UNCHANGED, except for (1) using the getListView() on
popup rather than accessing its mDropDownList member, and (2)
replacing ListPopupWindow return values with ShowableListMenu.

MenuAdapter:
Decouple from MenuPopupHelper so it may be used by forthcoming "menu
popup" implementations rather than just by MenuPopupHelper itself.
This class is UNCHANGED.

ShowableListMenu:
A new interface to encapsulate a menu that can be shown or hidden.
This will be implemented by both ListPopupWindow and a forthcoming
"menu popup" class.

Bug: 20127825
Change-Id: I565b444d25e966ff8b8c6ceac7be8de56a9116df