Fix brightness slider - allow minimum value

ag/11455081 stopped the brightness slider from reaching the minimum value. This change reverts this but other changes in this topic also change the Walleye initial curves to start at 1 instead of 0.

Bug: 157838219

Test: manual - cover ambient light sensor - ensure brightness reduces
but not to 0. Move the slider to min and check that it stays there. grep for brightness splines too:
adb shell dumpsys display | grep -i "spline\|mConfig"

Change-Id: I26334986d299d297e6d1e320222566b22497b2ff