Allow listener for native Animated.event

Summary:
We used to not send events to JS when there was a native Animated.event but we do now so we can easily enable the listener property.

**Test plan**
Tested that the listener gets called when using native Animated.event.
Closes https://github.com/facebook/react-native/pull/12323

Differential Revision: D4556407

fbshipit-source-id: 0b17f331a352d03a47f1611c667433fd5a58696c

Libraries/Animated/src/AnimatedImplementation.js

@@ -2162,10 +2162,6 @@ class AnimatedEvent {
     this._listener = config.listener;
     this.__isNative = shouldUseNativeDriver(config);
 
-    if (this.__isNative) {
-      invariant(!this._listener, 'Listener is not supported for native driven events.');
-    }
-
     if (__DEV__) {
       this._validateMapping();
     }
@@ -2215,6 +2211,10 @@ class AnimatedEvent {
   }
 
   __getHandler() {
+    if (this.__isNative) {
+      return this._listener;
+    }
+
     return (...args) => {
       const traverse = (recMapping, recEvt, key) => {
         if (typeof recEvt === 'number' && recMapping instanceof AnimatedValue) {