kiosk-shell: Separate (keyboard) input focus from xdg-shell toplevel window activation

This is small follow-up from !568 (closed) in which input focus was tied to window activation.

Note that these series has been split kiosk-shell (current MR) and desktop-shell (which have been migrated to !711 (closed))

This removes the window activation code from the keyboard focus handlers an embeds it straight into the shell's activation parts, such that devices with no keyboard can still use their other input devices (touch) to activate the window.