Only show that have sessions in fingerprint list

Doesn't access database directly anymore but goes through AxolotlService
now to obtain list of fingerprints associated with an Account/Contact.
This should prevent orphaned keys littering the UI which previously
couldn't be removed through the Clear Devices function.

Together with 1c79982da84964c1d81179a0927d9cd1eadf53de this fixes #1393