ucs: fix Persona key extraction
Based on the log provided in the following discussion thread
https://sourceforge.net/p/sipe/discussion/688534/thread/
db025884/
it turned out that the text for Attribution/DisplayName is localized,
i.e. the text "Lync Contacts" gets translated to the local language.
Switch over to check IsQuickContact == true and IsHidden == false, which
seems to be another indication that the Attribution node contains the
key for the Lync Contact. It's still a guessing game :-(
(cherry picked from commit
8763852bb5aa9709bbe6f93b9088f8d607e061b8)