conversationsview: always retrieve the correct best id

We used the registeredname as the best id, but it's not the case, we
need to get the best id from the registeredname or the uri.
note, remove useless TODO linked to the quality.

Change-Id: I1339d20ee1e4dd96c5c53b932b1fe262b6a53783
Reviewed-by: Nicolas Jäger <nicolas.jager@savoirfairelinux.com>