This is partially an emergency fix also for
https://invent.kde.org/plasma/plasma-mobile/-/issues/404, which was
potentially introduced by https://invent.kde.org/plasma/plasma-mobile/-/merge_requests/582
This MR extracts the screen brightness implementation from initialstart
to a QML plugin, and also ports the action drawer brightness slider
implementation to it. This avoids importing the powerdevil screen
brightness plugin on the lockscreen, which apparently has a race
condition that causes it to sometimes segfault.
