Control led brightness and color separately

Now that the mce libhybris plugin supports controlling led brightness
separately from led color, make mce utilize it.

[mce] Control led brightness and color separately. Fixes JB#19788
Use last known lux value for led brightness control

When the display is powered down also the ambient light sensor is
disabled. Since leds are mostly used when display is off, we need
to separate "ambient light sensor temporarily disabled" from "ambient
light sensor not available or permanently disabled" and use the last
known lux value when possible.
Merge pull request #172 from spiiroin/led_brightness

Control led brightness and color separately