iOS: Introduce API for moving screen reader's focus

Summary:
This change introduces an API, `setAccessibilityFocus`, which moves the screen reader's focus to the passed in element. This causes VoiceOver to announce the element and draw a focus rectangle around it.

Similar functionality is already available in RN Android through the `sendAccessibilityEvent` method. Here's an example of what exists today in RN Android:

```
RCTUIManager.sendAccessibilityEvent(
  node,
  8 /* TYPE_VIEW_FOCUSED */);
```

Called `setAccessibilityFocus` on a couple of elements to verify that focus does indeed move when VoiceOver is enabled. Additionally, my team is using this change in our app.

Adam Comella
Microsoft Corp.
Closes https://github.com/facebook/react-native/pull/14169

Differential Revision: D5137002

Pulled By: javache

fbshipit-source-id: 466e8b187e625de7c0f0d36e0400327dcd8d192a

Libraries/Components/AccessibilityInfo/AccessibilityInfo.ios.js

@@ -112,6 +112,15 @@ var AccessibilityInfo = {
     };
   },
 
+  /**
+   * iOS-Only. Set accessibility focus to a react component.
+   */
+  setAccessibilityFocus: function(
+    reactTag: number
+  ): void {
+    AccessibilityManager.setAccessibilityFocus(reactTag);
+  },
+
   /**
    * Remove an event handler.
    */