Additional fixes for button/keyboard backlight auto-brightness.

- Use sensor-determined brightness values when the button and/or keyboard
  lights are on, even if button/keyboard auto-brightness is disabled, since
  users may want to specify a custom brightness setting that disables the
  button/keyboard backlight entirely in low-ambient light situations.

- When button/keyboard auto-brightness is disabled, don't update button and
  keyboard brightnesses immediately if ambient light (the sensor) changes,
  as this may reactivate these lights after they've turned off in absense of
  user activity.  This is a particular problem when watching video or other
  circumstances when the lights are not expected to reactivate.