add rudimentary support for swapping using a hotspot

This adds rudimentary support for the swap procedure when one of the devices is a WiFi Access Point aka "hotspot". This reworks the procedure to get the wifi info, hopefully simplifying it, to make it also fetch the IP address from the hotspot.

This does not rework the UI at all, so when the device is a hotspot, JoinWifiActivity shows that there is no connection. But you can skip that screen, and it will work.

As far as I can tell, this does not change the existing functionality, just adds the rudimentary support for hotspots that we can build upon. @pserwylo @mvdan