Lock screen by default, on mobile

Summary:
At the moment it will do the same thing as the desktop: show the logout dialog
countdown, which doesn't make a lot of sense in this form factor.

Reviewers: #plasma, broulik, nicolasfella

Reviewed By: nicolasfella

Subscribers: nicolasfella, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D25908