Fix lint warning: Avoid non-default constructors in fragments

Summary: It is common practice to use a static newInstance to create new fragment instances instead of using non-default constructors

Reviewers: KDE Connect, nicolasfella

Reviewed By: KDE Connect, nicolasfella

Subscribers: kdeconnect

Tags: KDE Connect

Differential Revision: https://phabricator.kde.org/D17390