Remove ability to add contacts via telepathy plugin

Summary: Adding a contact in the desktop has the assumption that this contact will appear on the phone. I would like to make sure this assumption doesn't hold by having the plugin report that the contacts book is non-modifiable

Reviewers: kdeconnect, davidedmundson, apol

Reviewed By: apol

Subscribers: apol

Differential Revision: https://phabricator.kde.org/D5922