[upowerbackend] set cached brightness manually in case of led backlight

Summary:
In normal backlight subsystem's drivers typical workflow is,

• setBrightness gets called
• it writes to sysfs file using backlight helper
• kernel sends uevent for brightness change
• onDeviceChanged is triggered
• we update cache value

However in case of leds subsystem, kernel doesn't send uevent for
brightness change, hence uevent is never triggered and this doesn't
update cached value of brightness, resulting in DBus signal for
brightnessChanged never being emitted.

Test Plan: Tested on Nexus 5 which uses leds subsystem

Reviewers: Plasma, broulik

Reviewed By: Plasma, broulik

Subscribers: graesslin, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D2470