Don't show icons for top level menus unless forced.

Prior to this CL, the cascading implementation would evaluate the
contents of the menu to determine whether to show an icon for this case.
However, the policy we have upheld is that top level menus should
never show an icon for this case. (Unless setForceShowIcon(true) was
called.)

Bug: 28275346
Change-Id: I09423596d49742aa7a9ba7245702c099581c0b48