Solid-device-automounter/kcm: correctly update automountOn

Test Plan:
Open kcmshell5 device_automounter_kcm
Play with the checkbox "Automatically mount..." and see that the corresponding columns below are enabled/disabled.

Reviewers: bport, #plasma, ervin, crossi, hchain

Reviewed By: ervin

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27971