Add support for native animated events on iOS

Summary:
This adds native support for `Animated.event` on iOS.

**Test plan**
Tested in the native animated UIExplorer example that it works properly like on Android.
Closes https://github.com/facebook/react-native/pull/9598

Differential Revision: D4110331

fbshipit-source-id: 15748d23d0f475f2bcd1040ca3dca33e2620f058

Examples/UIExplorer/js/NativeAnimationsExample.js

@@ -191,6 +191,7 @@ class EventExample extends React.Component {
         <Animated.ScrollView
           horizontal
           style={{ height: 100, marginTop: 16 }}
+          scrollEventThrottle={16}
           onScroll={
             Animated.event([{
               nativeEvent: { contentOffset: { x: this.state.scrollX } }
@@ -462,6 +463,14 @@ exports.examples = [
       );
     },
   },
+  {
+    title: 'Animated events',
+    render: function() {
+      return (
+        <EventExample />
+      );
+    },
+  },
   {
     title: 'Internal Settings',
     render: function() {
@@ -470,13 +479,4 @@ exports.examples = [
       );
     },
   },
-  {
-    title: 'Animated events',
-    platform: 'android',
-    render: function() {
-      return (
-        <EventExample />
-      );
-    },
-  },
 ];