Don't trigger passkey pairing notification if remote device is unknown

The device address is essential for proper processing of
pairingDisplayConfirmation() signal on Android. If it is not available
abort immediately. This brings the passkey pairing code in line with
pincode pairing.

Change-Id: I174a2478d48463c58ba937e71ced377d45892f1a
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>