The brightness slider now always represents the absolute brightness,
including when adaptive brightness is enabled. This changes the summary
of the brightness setting to reflect that.
Bug: 72627086
Test: ROBOTEST_FILTER=BrightnessLevelPreferenceControllerTest m -j RunSettingsRoboTests
Change-Id: I76eed8aa9ee88fe6b70e30d54bfc0d18d1141377