Add support to completely disable key leds via onoff

onoff replaces state and supports three values:

- echo off > onoff
Completely turns off the key leds, but continues to save pattern changes.
Those are restored if the key leds are switched on again.

- echo on > onoff
Set the key leds state to dynamic mode.
That means, every time an input is set to on the leds are switched
dynamically on, and if all inputs are set to off the key leds are
switched dynamically off.

- echo reset > onoff
Switches the key leds off, clears all saved states and switches them on again.

Additionally some cleanup of the code.

Change-Id: I9694584b0d4fed9b54da0709ef5639f96b61d924