Fix edge case where screens were missing back button

When a root containing a stack with multiple children was displayed, initial stack children were
missing back button.

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

@@ -53,6 +53,7 @@ public class StackController extends ParentController<StackLayout> {
         for (ViewController child : children) {
             stack.push(child.getId(), child);
             child.setParentController(this);
+            if (size() > 1) backButtonHelper.addToPushedChild(child);
         }
     }
 

lib/android/app/src/test/java/com/reactnativenavigation/viewcontrollers/stack/StackControllerTest.java

@@ -98,6 +98,13 @@ public class StackControllerTest extends BaseTest {
         }
     }
 
+    @Test
+    public void constructor_backButtonIsAddedToChild() {
+        createStack(Arrays.asList(child1, child2, child3));
+        assertThat(child2.options.topBar.buttons.back.visible.get(false)).isTrue();
+        assertThat(child3.options.topBar.buttons.back.visible.get(false)).isTrue();
+    }
+
     @Test
     public void createView_currentChildIsAdded() {
         StackController uut = createStack(Arrays.asList(child1, child2, child3, child4));