feat(als): add flag indicating LED interference (#45)

on Diamond EVT, ALS sensor is located on the front unit, close
to the front LEDs. That ones are interfering with the ALS readings.
Make sure to mark the ALS reading as invalid if the front LEDs
are on.

messages/main.proto

@@ -598,6 +598,8 @@ message AmbientLight
         ALS_OK = 0;
         ALS_ERR_RANGE = 1; // likely too much light in the sensor, consider the
                            // value as ~500
+        ALS_ERR_LEDS_INTERFERENCE = 2; // front LEDs are turned on, interfering
+                                       // with ALS so value cannot be trusted
     };
     uint32 ambient_light_lux = 1;
     Flags flag = 2;