fix StatusBar showHideTransition flow type issue (#29034)

Summary:
This small PR adds third, missing [`StatusBarAnimation`](https://github.com/facebook/react-native/blob/master/Libraries/Components/StatusBar/StatusBar.js#L45) possible value to the `showHideTransition` prop validation - `'none'`.

This fixes the following issue when `<StatusBar showHideTransition="none" />` code  is used:

<img width="970" alt="Screenshot 2020-06-02 at 22 39 37" src="https://user-images.githubusercontent.com/719641/83567510-f6b85200-a521-11ea-9f1c-48825d0285bf.png">

## Changelog

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[iOS] [Fixed] - fix StatusBar showHideTransition flow type issue

Pull Request resolved: #29034

Test Plan: I have run the `flow` check in the test project which uses local working copy of `react-native` with this change included.

Reviewed By: GijsWeterings

Differential Revision: D22493825

Pulled By: cpojer

fbshipit-source-id: 463badec67e6725cb8711a79aa43d84f9b09f796

Libraries/Components/StatusBar/StatusBar.js

@@ -87,7 +87,7 @@ type IOSProps = $ReadOnly<{|
    *
    * @platform ios
    */
-  showHideTransition?: ?('fade' | 'slide'),
+  showHideTransition?: ?('fade' | 'slide' | 'none'),
 |}>;
 
 type Props = $ReadOnly<{|