Fix Tpf.Persona to use its store's ID in persona UIDs

It was previously (incorrectly) not using the PersonaStore's ID as the middle
component in its UID, which meant that the same IM address as added on two
different accounts using the same protocol would have the same UID,
undermining the concept of a UID as a globally unique identifier for a
Persona.

Closes: bgo#647121