MenuPopupHelper: Factor out a MenuPopup interface.

Move much of the responsibility into implementations of this interface.
Delegate functionality to it where appropriate.
Provide a standard (non-cascading) implementation for this interface.

This CL should have NO BEHAVIOR CHANGES.

A follow-up CL will provide a cascading implementation, whereby a
config variable will enable submenus to open side by side with their
parent menus. That CL will be the first with functional/ actual behavior
changes.

Bug: 20127825
Change-Id: Iecac2d340dd8750ebe4e99162d447c9411f09227