Fix NPE when opening Google Search Assist when a RN Dialog is displayed

Summary:
This fixes app crashes on opening android search assistant when a RN modal is visible. The root cause is that ViewGroup.dispatchProvideStructure() method uses the private variable 'mChildren' to access the children of a view group instead of the publicApi.

This fixes the top crash for the react_MarketplaceProductDetailsNonIPadRoute route.

This also fixes github issues:
facebook#15932
facebook#13201
facebook#15440

that were closed without a fix

Reviewed By: PeteTheHeat

Differential Revision: D13375993

fbshipit-source-id: d603cb4ef65a423c63a6ef2b51235702c7dbffcb

Author: mdvacca

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

@@ -7,6 +7,7 @@
 
 package com.facebook.react.views.modal;
 
+import android.annotation.TargetApi;
 import android.app.Activity;
 import android.app.Dialog;
 import android.content.Context;
@@ -15,6 +16,7 @@
 import android.view.MotionEvent;
 import android.view.View;
 import android.view.ViewGroup;
+import android.view.ViewStructure;
 import android.view.WindowManager;
 import android.view.accessibility.AccessibilityEvent;
 import android.widget.FrameLayout;
@@ -73,6 +75,12 @@ public ReactModalHostView(Context context) {
     mHostView = new DialogRootViewGroup(context);
   }
 
+  @TargetApi(23)
+  @Override
+  public void dispatchProvideStructure(ViewStructure structure) {
+    mHostView.dispatchProvideStructure(structure);
+  }
+
   @Override
   protected void onLayout(boolean changed, int l, int t, int r, int b) {
     // Do nothing as we are laid out by UIManager