Fix a bug where clicking the "more results" suggestion with the trackball

did not seem to expand to show the corpus selectors. (http://b/1906643)

It turns out that when the trackball was used to select this item, onItemClick
was getting called on the listener twice in AutoCompleteTextView#onKeyUp:
once when we call onKeyUp on the DropDownListView, and once a little further
down when we call performCompletion(). In our case, performCompletion() is
useless since it was intended for normal AutoCompleteTextViews which populate
text into the TextView when an item in the list is clicked, but we do not
rely on this. So the easiest fix is just to make performCompletion() do nothing
in our SearchAutoComplete subclass.