[backlight helper] Support setting backlight through leds subclass

Summary:
Some devices uses simple leds to provide the backlight to display, one
such example is Nexus 5, which uses /sys/class/leds/lcd-backlight/.

Reviewers: broulik, Plasma

Reviewed By: broulik, Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D2419