Set Activity contentView in setRoot

This allows users to set splash layouts in the Activities' onCreate method
by passing their splash view to setContentView in onCreate.

```java
public class MainActivity extends NavigationActivity {

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        View splash = new View(this);
        splash.setBackgroundColor(Color.RED);
        setContentView(splash);
    }
}
```

lib/android/app/src/main/java/com/reactnativenavigation/NavigationActivity.java

@@ -31,7 +31,6 @@ public class NavigationActivity extends AppCompatActivity implements DefaultHard
         super.onCreate(savedInstanceState);
         navigator = new Navigator(this, new ChildControllersRegistry(), new ModalStack(this), new OverlayManager());
         getReactGateway().onActivityCreated(this);
-        setContentView(navigator.getView());
     }
 
     @Override

lib/android/app/src/main/java/com/reactnativenavigation/viewcontrollers/Navigator.java

@@ -105,6 +105,9 @@ public class Navigator extends ParentController {
 
     public void setRoot(final ViewController viewController, CommandListener commandListener) {
         destroyRoot();
+        if (view == null) {
+            getActivity().setContentView(getView());
+        }
         root = viewController;
         contentLayout.addView(viewController.getView());
         if (viewController.options.animations.startApp.hasAnimation()) {

lib/android/app/src/test/java/com/reactnativenavigation/viewcontrollers/NavigatorTest.java

@@ -88,7 +88,6 @@ public class NavigatorTest extends BaseTest {
         child3 = new SimpleViewController(activity, childRegistry, "child3", tabOptions);
         child4 = new SimpleViewController(activity, childRegistry, "child4", tabOptions);
         child5 = new SimpleViewController(activity, childRegistry, "child5", tabOptions);
-        activity.setContentView(uut.getView());
 
         activityController.visible();
     }
@@ -105,9 +104,15 @@ public class NavigatorTest extends BaseTest {
         verify(modalStack).setDefaultOptions(defaultOptions);
     }
 
+    @Test
+    public void setRoot_setContentViewWhenFirstRootIsSet() {
+        assertThat(uut.view).isNull();
+        uut.setRoot(child1, new CommandListenerAdapter());
+        assertThat(uut.view).isNotNull();
+    }
+
     @Test
     public void setRoot_AddsChildControllerView() {
-        assertThat(uut.getContentLayout().getChildCount()).isZero();
         uut.setRoot(child1, new CommandListenerAdapter());
         assertIsChild(uut.getContentLayout(), child1.getView());
     }