Fix brightness curve when user sets slider to min.

The brightness spline was setting the entire curve to 0.0f. This was due
to the min and max points of point x+1 being constrained to a factor of
the brightness x. If this happens to be 0.0f, the brightness curve will
never increase.

Bug: 153799222

Test: AutomaticBrightnessControllerTest

Change-Id: Id29389e7fc17b402d2f1bd581a805a6067eac4bb