When connecting to a new device, a random key gets generated, which can be
looked up from the device specific settings (accessible via the gear icon in
the device card in the main activity). Old devices keep their 0123456789@ABCDE
key, they have to be re-paired to change that.
During pairing, long-pressing the device candidate in the discovery activity
will also start the device specific settings activity, where the auth key
can be set manually priror to pairing. This is usefull to keep the ability to
pair one device with multiple android devices.
Fixes#1308
