Display: Add brightness_direction field to DisplayBrightnessChanged atom Introduces a new 'brightness_direction' field to the 'DisplayBrightnessChanged' atom. This change also adds a 'BrightnessDirectionEnum' to define the possible outcomes when a brightness adjustment is made. Test: StatsD and manual (using device) Bug: 291013376 Flag: EXEMPT no API change Change-Id: I937e609d3e94d8a3b62d6c3227b9172a44d77883