Use spline interpolation for auto-brightness.

Strictly speaking, this is a change in behavior for all products.
Instead of using discrete zones, they will all now use spline
interpolation.  We could make this behavior configurable
but there seems to be little point to it.  The range of brightness
values used will be more or less the same as before, it's just
that what used to be the brightness value for all levels within
a particular zone now becomes the brightness value for the
highest level in that zone and lower values are used for lower
levels within the zone.

Change-Id: I39804ee630ba55f018e1e53c0576b28e7bd27931