Fix BrightnessTracker's handling of color temperature

If the user was using the default color temperature
for nightmode BrightnessTracker would incorrectly
record it as 0.

Bug: 112626811
Bug: 113656622
Test: atest BrightnessTrackerTest
Change-Id: Ib9152f516d0f2b24e1a3f91748f7ccc1647db222