Improve search contacts features

[Sosthene00]

We identified UX and security issues with the search contacts feature:

• User could search for contacts they registered and other contacts from the name server at the same place without sufficiently clear distinction
• Add contact form didn't allow searching for unregistered addresses from the name server

That PR cleanly separates the 2 features (searching our contacts vs searching public addresses):

• ContactsScreen only search for contacts already registered in our device
• AddContactSheet requests name server and returns contacts we haven't registered yet