MDL-68167 lib: Fix label for autocomplete selection/suggestion box

MDL-68167 lib: Fix label for autocomplete selection/suggestion box

The content of the autocomplete selection div is purged by javascript
so we have to move its label outside of it. aria-labeledby is the way
to go.

Each time the suggestion box is expanded, autocomplete js renders the
autocomplete suggestions template. So the label cannot be outside the
suggestions ul.