Support devices with amber/green LEDs.

On devices like the Legend HTC used a binary amber/green LED with
support for hardware blinking.
Both colors on at the same time result in a slightly lighter amber that
is not distinguishable from amber at all. So we have to make sure their
status is mutual exclusive.

I decided to use amber as charging and battery low indicator and green
for notifications. Green always overrides amber.

Change-Id: Id4e8a1cd7a2be217b5cf4c5b45b1fc5beba5d2db