Fix crash on startup on Android.

Summary:
_ZN22SolidBrightnessBackendC1EP7QObject symbol not found.

Commit 1466a25 removed compilation of solidbrightnessbackend.cpp on
Android, so this code now uses a class that's not part of the build.

Test Plan: Running on Android now works

Reviewers: vkrause, nicolasfella

Reviewed By: vkrause

Differential Revision: https://phabricator.kde.org/D17763