Expose StatusBar height on iOS

Summary:
Following the PR https://github.com/facebook/react-native/pull/6195, this adds a `HEIGHT` constant on `StatusBar` for iOS.

Combined with `statusBarFrameDidChange` and `statusBarFrameWillChange` StatusBar native events, it solves various problems with In-Call cellar bar / Location bar / others 40pt status bars, and offers a correct `keyboardVerticalOffset` value for the KeyboardAvoidingView component.
Closes https://github.com/facebook/react-native/pull/12041

Differential Revision: D4450924

Pulled By: hramos

fbshipit-source-id: 664798260f4226140f3fa3f9222a415a305d0d78

Examples/UIExplorer/js/StatusBarExample.js

@@ -438,11 +438,10 @@ const examples = [{
   render() {
     return (
       <View>
-        <Text>Height (Android only): {StatusBar.currentHeight} pts</Text>
+        <Text>Height: {StatusBar.currentHeight} pts</Text>
       </View>
     );
   },
-  platform: 'android',
 }];
 
 exports.examples = examples;