Make key-above-tab in switcher more consistent

Ensure that key-above-tab for selecting window application is consistent
with the down key.
After focus an application in the switcher:
- if you press down, the window thumbnails are previewed, and first element
  is selected
- if you press key-above-tab, the window thumbnails are previewed, however
  the second element is directly selected.
Make both interactions always select the first element.

https://bugzilla.gnome.org/show_bug.cgi?id=786009