Bluetooth address and UUID aren't stored until contact connections are enabled

The fix for #1001 (closed) caused a regression: the Bluetooth address and UUID aren't stored in the local transport properties until contact connections are enabled. This causes the following issue:

Turn off wifi
Start Briar with a fresh account, add a contact via Bluetooth
Messages can be exchanged with the contact
Sign out and back in
Enable Bluetooth contact connections in Briar settings
Bluetooth plugin can't connect to contact because it doesn't have contact's address or UUID

On the beta branch, which doesn't have the fix for #1001 (closed), the Bluetooth plugin connects in the last step.