Bluetooth setup screen

Reflected transport properties should enable most devices to learn their own Bluetooth addresses, but there's still a bootstrapping problem when a group of two or more contacts are all running Android 8.1+ and none of them added each other via Bluetooth.

We could solve this by adding a "Bluetooth setup" workflow. If the user activates the Bluetooth plugin and we don't know the device's address, we tell the user that Bluetooth needs to be set up and that they need a contact nearby to help with this. When both users are ready, we perform discovery, show a list of devices, and ask the users to pick each other's devices. If the devices succeed in making a connection they'll learn each other's addresses and exchange them via reflected transport properties.