Adding additional check to if shutdown is already in process

lib/devices/android/android.js

@@ -363,6 +363,10 @@ Android.prototype.startApp = function (cb) {
 };
 
 Android.prototype.stop = function (cb) {
+  if (this.shuttingDown) {
+    logger.debug("Already in process of shutting down.");
+    return cb();
+  }
   this.shuttingDown = true;
 
   var completeShutdown = function (cb) {