Make AccessibilityInfo.setAccessibilityFocus cross platform (#20229)

Summary:
Currently, `AccessibilityInfo.setAccessibilityFocus` is only available on iOS. The same behaviour can be achieved on Android by dispatching the proper accessibility event. I implemented the same function for Android, to make life slightly more convenient for the developer.

Today, developers must write something like this:
```
if (Platform.OS === 'ios') {
     AccessibilityInfo.setAccessibilityFocus(reactTag)
} else {
     UIManager.sendAccessibilityEvent(reactTag, 8)
}
```

With this change, the following is enough for both Android and iOS:
```
AccessibilityInfo.setAccessibilityFocus(reactTag)
```
Pull Request resolved: https://github.com/facebook/react-native/pull/20229

Differential Revision: D8874107

Pulled By: mdvacca

fbshipit-source-id: a6ffd7bb89ce56d6d65b06419633a71dcf3d0733

Libraries/Components/AccessibilityInfo/AccessibilityInfo.android.js

@@ -12,6 +12,7 @@
 
 const NativeModules = require('NativeModules');
 const RCTDeviceEventEmitter = require('RCTDeviceEventEmitter');
+const UIManager = require('UIManager');
 
 const RCTAccessibilityInfo = NativeModules.AccessibilityInfo;
 
@@ -66,6 +67,18 @@ const AccessibilityInfo = {
     listener.remove();
     _subscriptions.delete(handler);
   },
+
+  /**
+   * Set accessibility focus to a react component.
+   *
+   * See http://facebook.github.io/react-native/docs/accessibilityinfo.html#setaccessibilityfocus
+   */
+  setAccessibilityFocus: function(reactTag: number): void {
+    UIManager.sendAccessibilityEvent(
+      reactTag,
+      UIManager.AccessibilityEventTypes.typeViewFocused,
+    );
+  },
 };
 
 module.exports = AccessibilityInfo;

Libraries/Components/AccessibilityInfo/AccessibilityInfo.ios.js

@@ -93,8 +93,6 @@ const AccessibilityInfo = {
   /**
    * Set accessibility focus to a react component.
    *
-   * @platform ios
-   *
    * See http://facebook.github.io/react-native/docs/accessibilityinfo.html#setaccessibilityfocus
    */
   setAccessibilityFocus: function(reactTag: number): void {

ReactAndroid/src/main/java/com/facebook/react/uimanager/UIManagerModuleConstants.java

@@ -134,6 +134,8 @@ import java.util.Map;
       MapBuilder.of(
           "typeWindowStateChanged",
           AccessibilityEvent.TYPE_WINDOW_STATE_CHANGED,
+          "typeViewFocused",
+          AccessibilityEvent.TYPE_VIEW_FOCUSED,
           "typeViewClicked",
           AccessibilityEvent.TYPE_VIEW_CLICKED));