Don't make or accept Bluetooth contact connections during key agreement

This branch improves the success rate of Bluetooth key agreement connections by closing all Bluetooth connections when key agreement starts, and not making or accepting Bluetooth contact connections during key agreement.

When multiple contacts are added via Bluetooth, this increases the chances of adding all contacts successfully, at the cost of disconnecting from all but the most recently added contact. I hope to improve this tradeoff in the future by detecting how many simultaneous connections the device can reliably support.