Keep the adjusted and unadjusted spline in BrightnessMappingStrategy

When RBC is applied, the spline in BMS is adjusted. In these situations, we should use the unadjusted spline for nit conversion:
- persisting the nits value between displays (ag/21508452): it's easier to store the unadjusted nit value than the adjusted one and changing it when RBC is turned off
- passing the nit value to follower displays: we should let each display apply its own adjustments
In all other situations, we continue to use the adjusted spline.

Bug: 273650471
Test: atest BrightnessMappingStrategyTest
Test: atest DisplayBrightnessControllerTest
Test: atest DisplayPowerControllerTest
Test: atest DisplayPowerController2Test
Change-Id: I3853feab7974163aff3ea5e7cd80cc5a8e307d15