[platforms/hwcomposer] Reset old brightness when turning screen back on

Summary:
Now powerdevil can adjust brightness using leds subsystem, however, kwin
as well sets brightness to 0 for turning off screen and 0xff when
turning screen back on. This resets the brightness set by the powerdevil
to 100%.

As a solution now kwin listens to brightnessChanged dbus signal of
brightnesscontrol and book-keeps the changed brightness, if screen is
turned off it sets brightness to 0 and when turning screen on, it resets
to old brightness.

If powermanagement service doesn't appear on dbus by default it restores
100% brightness.

Test Plan:
Appearantly this still doesn't work as-it-is on phone, because powerdevil
doesn't emit brightnessChanged dbus signal because of bug in the driver of
backlight control, driver doesn't seem to trigger uevents for changes in
backlight. But with hack in powerdevil to emit brightnessChanged when setting
brightness, this works

Reviewers: broulik, Plasma on Wayland, graesslin

Reviewed By: Plasma on Wayland, graesslin

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D2468