ModalHostView's child view can not fill screen (#18615)

Summary:
Signed-off-by: yukai <[email protected]>

In some Android device such as Samsung S8, the ModalHostView's child view can't fill the screen.
before:
![before](https://user-images.githubusercontent.com/1514899/38024961-3e756f8e-32b9-11e8-9555-50a7cf778288.jpeg)
The JS ModalHostView can't fill the bottom, the area is device's navigation bar.

In class ModalHostShadowNode, follow code calculate is error:
`Point modalSize = ModalHostHelper.getModalHostSize(getThemedContext());`
This way not care the device's navigation bar, so the height is smaller than real.
For Samsung S8, real height is 2220 and modalSize.y is 2076.
Pull Request resolved: #18615

Differential Revision: D14206830

Pulled By: cpojer

fbshipit-source-id: abe35ce1ab253aa1472d2c798543b515218be445

ReactAndroid/src/main/java/com/facebook/react/views/modal/ReactModalHostView.java

@@ -327,6 +327,9 @@ private void updateProperties() {
    * UIManagerModule, and will then cause the children to layout as if they can fill the window.
    */
   static class DialogRootViewGroup extends ReactViewGroup implements RootView {
+    private boolean hasAdjustedSize = false;
+    private int viewWidth;
+    private int viewHeight;
 
     private final JSTouchDispatcher mJSTouchDispatcher = new JSTouchDispatcher(this);
 
@@ -337,17 +340,34 @@ public DialogRootViewGroup(Context context) {
     @Override
     protected void onSizeChanged(final int w, final int h, int oldw, int oldh) {
       super.onSizeChanged(w, h, oldw, oldh);
+      viewWidth = w;
+      viewHeight = h;
+      updateFirstChildView();
+    }
+
+    private void updateFirstChildView() {
       if (getChildCount() > 0) {
+        hasAdjustedSize = false;
         final int viewTag = getChildAt(0).getId();
         ReactContext reactContext = getReactContext();
         reactContext.runOnNativeModulesQueueThread(
           new GuardedRunnable(reactContext) {
             @Override
             public void runGuarded() {
               (getReactContext()).getNativeModule(UIManagerModule.class)
-                .updateNodeSize(viewTag, w, h);
+                .updateNodeSize(viewTag, viewWidth, viewHeight);
             }
           });
+      } else {
+        hasAdjustedSize = true;
+      }
+    }
+
+    @Override
+    public void addView(View child, int index, LayoutParams params) {
+      super.addView(child, index, params);
+      if (hasAdjustedSize) {
+        updateFirstChildView();
       }
     }