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.
    */

React/Modules/RCTAccessibilityManager.m

@@ -9,6 +9,7 @@
 
 #import "RCTAccessibilityManager.h"
 
+#import "RCTUIManager.h"
 #import "RCTBridge.h"
 #import "RCTConvert.h"
 #import "RCTEventDispatcher.h"
@@ -162,6 +163,14 @@ RCT_EXPORT_METHOD(setAccessibilityContentSizeMultipliers:(NSDictionary *)JSMulti
   self.multipliers = multipliers;
 }
 
+RCT_EXPORT_METHOD(setAccessibilityFocus:(nonnull NSNumber *)reactTag)
+{
+  dispatch_async(dispatch_get_main_queue(), ^{
+    UIView *view = [self.bridge.uiManager viewForReactTag:reactTag];
+    UIAccessibilityPostNotification(UIAccessibilityLayoutChangedNotification, view);
+  });
+}
+
 RCT_EXPORT_METHOD(getMultiplier:(RCTResponseSenderBlock)callback)
 {
   if (callback) {