emacs: Fix 's' keybinding to go to search bar in notmuch-hello.

emacs: Fix 's' keybinding to go to search bar in notmuch-hello.

This command was previously written under the fragile assumption that
the search bar was always the third widget. That's no longer true with
the saved searches now appearing before the search bar, so we save the
position of the search bar and go directly to it now.