Add window manager support for overriding button and keyboard backlight values.

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