Cascading popup menus: open submenu on mouse hover.

When the cascading feature is enabled, users can mouseover a
submenu item in a popup menu to expand and open the new submenu
(after a short timeout). Similarly, if a user mouseovers a
different menu item in the original menu, the submenu gets closed
(again, after a short timeout).

This should complete the implementation of cascading submenu
functionality.

Also fix two other issues:

(1) Update some oudated code in PopupMenu that was still opening
the submenu when a user clicks on a submenu item; this
responsibility now lives within the MenuPopupHelper's delegate
MenuPopup class, so it doesn't need to live in PopupMenu anymore.

(2) Fix an issue where icons would be force-set on a submenu when they
should not be. Instead, decide whether to show icons in a submenu
based on whether to show them in the top level menu, as intended.

Bug: 20127825
Change-Id: Ia46852c7f99436065ab4bc234de94dffc0019666