sdk: Allow controllable brightness for non-RGB segmented battery LEDs

* For non-RGB segmented battery LEDs, we currently don't allow any kind
  of brightness control, because the alpha channel is taken up by the level
* To remedy this, set the brightness in the color channel like we do for
  setting panel backlight brightness by setting the brightness as R/G/B

Change-Id: I4e47861643e0b2b8766af0f2ff275069fc580108