Prevent PowerDevil from stopping UPower service

This is a workaround for Bug 486067 until a proper fix is merged.

Credits to Thomas Baag for proposing the fix in Bug 487743.

This issue started with commit fe9e90f5

Before this patch:

$ journalctl -b -u upower.service
Feb 08 15:29:10 bespin systemd[1]: Starting Daemon for power management...
Feb 08 15:29:10 bespin systemd[1]: Started Daemon for power management.
Feb 08 15:30:35 bespin systemd[1]: Stopping Daemon for power management...
Feb 08 15:30:35 bespin systemd[1]: upower.service: Deactivated successfully.
Feb 08 15:30:35 bespin systemd[1]: Stopped Daemon for power management.

After this patch:

$ journalctl -b -u upower.service
Feb 08 15:31:21 bespin systemd[1]: Starting Daemon for power management...
Feb 08 15:31:21 bespin systemd[1]: Started Daemon for power management.