Cascading submenus: Fix a few small issues.

1. Open submenus now disappear instantly rather than fading out when
mouse hovers over a non-submenu item.

2. When a submenu is opened on hover, the item that was hovered now
will remain selected instead of being deselected.

3. Fix an issue where if the same chain of submenus was opened more
than once, the submenus could open in a different direction each
time. To fix, we keep track of the direction of opening along the way
and make sure to reset the preferred direction when a submenu is
closed.

Bug: 20127825
Change-Id: I99972cce4ddabe316c5148f1dd79a5d7b3e64b87