Fix slider being at 0 on unlock

This was happening due to a change that sourced the brightness value
from the brightnessInfo object, which includes brightness state
(invalid and -1/off) as well.

This change ensures the brightnessInfo object will only store valid
brightness values (ie, when the display is ON, and therefore the
brightness is between min & max).

Bug: 187776200
Test: manual
Change-Id: I6b7077da1354f0b21c570f4e2283aa6830dbdb93