feat: expose AnimatedEvent (#22984)

Summary:
Some third party components rely on the `AnimatedEvent` as a type (see https://github.com/kmagiera/react-native-gesture-handler/blob/master/Swipeable.js#L9). In order to not rely on a path which could change at any time it would be great to expose it.

Changelog:
----------

[General][added] - Expose AnimatedEvent
Pull Request resolved: https://github.com/facebook/react-native/pull/22984

Differential Revision: D13682678

Pulled By: cpojer

fbshipit-source-id: 2f2b228e3f49e6afe623fd551220ef2147e79550

Libraries/Animated/src/AnimatedEvent.js

@@ -158,7 +158,7 @@ class AnimatedEvent {
     };
   }
 
-  _callListeners(...args) {
+  _callListeners(...args: any) {
     this._listeners.forEach(listener => listener(...args));
   }
 

Libraries/Animated/src/AnimatedImplementation.js

@@ -685,5 +685,10 @@ module.exports = {
   forkEvent,
   unforkEvent,
 
+  /**
+   * Expose Event class, so it can be used as a type for type checkers.
+   */
+  Event: AnimatedEvent,
+
   __PropsOnlyForTests: AnimatedProps,
 };