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

Libraries/Components/StatusBar/StatusBar.js

@@ -134,7 +134,7 @@ function createStackEntry(props: any): any {
  *
  * ### Constants
  *
- * `currentHeight` (Android only) The height of the status bar.
+ * `currentHeight` The height of the status bar.
  */
 class StatusBar extends React.Component {
   props: {
@@ -169,8 +169,6 @@ class StatusBar extends React.Component {
   // discussion in #6195.
   /**
    * The current height of the status bar on the device.
-   *
-   * @platform android
    */
   static currentHeight = StatusBarManager.HEIGHT;