Track brightness changes in nits rather than backlight values.

For the moment, the only one that actually knows the true brightness
value is the DisplayPowerController, so have that tell the
BrightnessTracker directly when brightness changes.

Bug: 69405990
Test: atest com.android.server.display.BrightnessTrackerTest
      atest com.android.server.display.BrightnessMappingStrategyTest

Change-Id: Ibf4e501ce2f7b071360bfac501dbbafb3ba55fa5