added unregisterReceiver(usbReceiver) to onDestroy()

examplesync/src/main/java/com/felhr/serialportexamplesync/UsbService.java

@@ -156,6 +156,7 @@ public class UsbService extends Service {
     @Override
     public void onDestroy() {
         super.onDestroy();
+        unregisterReceiver(usbReceiver);
         UsbService.SERVICE_CONNECTED = false;
     }