DWB Bias splines ignore default ambient CCT

When ambientColorTemperature is set to a default of -1.0f, ambient
brightness to bias splines should ignore this values instead of trying
to interpolate to towards an actual CCT.

Bug: 132047710
Test: atest AmbientLuxTest
Change-Id: Ic6da5f80d52a94a29795a76422cc1b964cdf261b