Exclude powerdevil from session management

Summary:
I found this process in my ksmserverrc

This is autostarted through other ways, and there's no useful data that
should persist across sessions.

Test Plan: Code is copy pasted

Reviewers: #plasma, broulik

Reviewed By: #plasma, broulik

Subscribers: broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D19024