Fix 404 contact not found with shared addressbook

There are again 404 occuring, this time with a shared addressbook. The contacts app tries to search for a contact in a `local` addressbook, but it fails because the addressbook isn't owned by this user. Instead it should access the same addressbook but not by using the `local` backend but the `shared` backend.

Refs https://github.com/owncloud/chat/issues/159#issuecomment-63945487

Note that this solution is a hack! In the DB there isn't a field which backend is used for this contact, thus we should guess this. Luckily the contact id is unique -> there can't be a match in the local addressbook when the contact is stored in the shared addressbook.
Indeed, thanks!