MDCMenuFoundation add --selected to list items, so that states code is applied to menu

[lynnmercier]

Currently our mdc-menu does not show the selected state of a list-item.

This may need to be implemented in a variant of mdc-menu. Some menus, once an item is selected, the menu is never expected to be re-opened. But for mdc-select, the menu we attach to the select component can be re-opened. Right now we use [aria-selected="true"] instead of mdc-list-item--selected in our CSS selectors within mdc-select.