Change automatic brightness summary to absolute brightness.

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