Move brightness color conversion from SysUI to settingslib

The color space conversion logic in BrightnessController is also needed
by Settings app to display brightness percentage correctly. This change
moves the conversion logic to settingslib so it can be shared by other
UIs such as Settings/VrSettings.

Bug: 73810208
Test: atest RunSettingsLibRoboTests
Change-Id: If92f0da2d930fec797902b5d1b3731100e1d0ea4