Slider always represents absolute brightness

Currently the slider represents a gamma adjustment to the brightness
curve when auto brightness is enabled and the absolute screen brightness
when it's not. This is a fairly confusing behavior to most people, so
this consolidates them to a single behavior: the slider always
represents the current brightness and auto-brightness will automatically
adjust it.

This also moves a bunch of the brightness methods from PowerManager over
to DisplayManager, since it's really the DisplayPowerController that's
responsible for determining and setting the display brightness.

Test: atest com.android.server.display.BrightnessMappingStrategyTest
Bug: 69406898
Change-Id: I73b5982809a94cd50d563426a72d7965e923c994