Define brightness curve in backlight in DDC

The mapping is in the backlight scale, usually [0, 1]. The backlight scale is defined by the first and the last value in screenBrightnessMap in the DDC.

The nit mapping wasn't used and will be removed for now.

Bug: 293613040
Test: atest com.android.server.display
Change-Id: I88219b02c7e8c13491b453661d91a634a317f8e8