Add back button to last child in setStackRoot

This effectively reverts 96661e3308 and fixes the issue properly.

lib/android/app/src/main/java/com/reactnativenavigation/viewcontrollers/stack/StackController.java

                 }
             });
         } else {
+            backButtonHelper.addToPushedChild(last(children));
             push(last(children), new CommandListenerAdapter() {
                 @Override
                 public void onSuccess(String childId) {
                     destroyStack(stackToDestroy);
-                    for (int i = 0; i < children.size(); i++) {
+                    for (int i = 0; i < children.size() - 1; i++) {
                         stack.set(children.get(i).getId(), children.get(i), i);
                         children.get(i).setParentController(StackController.this);
                         if (i == 0) {