replace CupertinoPageScaffold with Material in CountrySelectPage (#1276)

Author: britannio

client/flutter/lib/pages/onboarding/country_select_page.dart

@@ -19,8 +19,8 @@ class CountrySelectPage extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    return CupertinoPageScaffold(
-      backgroundColor: CupertinoColors.white,
+    return Material(
+      color: CupertinoColors.white,
       child: Column(
         crossAxisAlignment: CrossAxisAlignment.stretch,
         children: <Widget>[