Fix no buttons bug for first screen

Due to race condition between SetupToolbarButtonsTask and
onCreateOptionsMenu buttons did not get set for first pushed screen.
In order to set buttons we need a ref to Menu which is set in
onCreateOptionsMenu. Sometimes onCreateOptionsMenu got called after
onCreate of our activity where we setup the toolbar.

android/app/src/main/java/com/reactnativenavigation/activities/BaseReactActivity.java

     @Override
     public boolean onCreateOptionsMenu(Menu menu) {
         mMenu = menu;
+        if (mToolbar != null) {
+            mToolbar.setupToolbarButtonsAsync(getCurrentScreen());
+        }
         return super.onCreateOptionsMenu(menu);
     }