Remove specificity on Android and iOS (#14282)

We run on more than two platforms these days.

README.md

@@ -3,8 +3,8 @@ Flutter Engine
 [![Build Status - Cirrus][]][Build status]
 
 Flutter is Google's mobile app SDK for crafting high-quality native interfaces
-on iOS and Android in record time. Flutter works with existing code, is used by
-developers and organizations around the world, and is free and open source.
+in record time. Flutter works with existing code, is used by developers and
+organizations around the world, and is free and open source.
 
 The Flutter Engine is a portable runtime for hosting
 [Flutter](https://flutter.dev) applications.  It implements Flutter's core