Hide OSK when a HW keyboard is present

Uses phoc-device-state unstable-v1 protocol to detect HW keyboard presence.

I just ripped off Guido's MR here: guidog/phosh-osk-stub!130 (merged)

I opted to not bring across the "HwTracker" pattern because a) I'm lazy and b) the ServerContextService is already a GObject thingie that is chiefly tasked with deciding if the OSK should show up or not, so it seemed appropriate to just hack the new protocol usage in there and call it a day.