WinRT: device discovery requires device pairing

While finding unpaired devices works on WinRT, the device discovery agent asks the user to pair the devices in order to be able to get its service GUIDs. The LEAdvertisementWatcher API allows us to obtain a list of service GUIDs though so that we might be able to obtain the needed information without pairing (the pairing request should be done as soon as the device is "used" in this case).