The UUID not unique per user in the telemetry data

Qt Creator Telemetry plugin uses a random UUID to associate several session records to one anonymous user. Even though the identifier is stored into settings, it looks like every time a data source object is created, a new identifier is created, so it's impossible to know the number of users or associate different records to one user. 

Instead of creating a new one, the one stored in the settings should be used as the UUID in the ServiceSource class.