[logind] Correct property name to fix logind session

Summary:
In 6ebc3d65f03 a small naming mistake led to Wayland session with
logind not working anymore. Patch to make it work again.

Also rename the variables because what they describe is a state
and not an action.

Reviewers: KWin

Subscribers: KWin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D6659