The new backlightBrightness field works similarly as the existing WindowManager.LayoutParams.screenBrightness field
Needed for bugs:
b/2233655 (under low ambient light the touch keys remain illuminated during video playback and never timeout)
b/2221079 (Backlight for home/search/back/etc buttons should turn off when in dock in night mode)
Change-Id: I60dfecdc7bb653b0db38094464de651220b3d438
fb73f79340