Narrow check for state change in led driver

Fixes use of "LED.OFF" state in status_ui state map

src/aiy/_drivers/_led.py

@@ -90,7 +90,7 @@ def _animate(self):
                 running = self.running
             if not running:
                 return
-            if state:
+            if state is not None:
                 if not self._parse_state(state):
                     raise ValueError('unsupported state: %d' % state)
             if self.iterator: