Animated: Implement toJSON method of Animated

Summary:
Fix issue https://github.com/facebook/react-native/issues/7441
Closes https://github.com/facebook/react-native/pull/7442

Differential Revision: D3274594

fb-gh-sync-id: 720d7dcdaa97a2ee5235547bad6ba020398b57d2
fbshipit-source-id: 720d7dcdaa97a2ee5235547bad6ba020398b57d2

Libraries/Animated/src/AnimatedImplementation.js

@@ -69,6 +69,7 @@ class Animated {
   __getNativeConfig(): Object {
     throw new Error('This JS animated node type cannot be used as native animated node');
   }
+  toJSON(): any { return this.__getValue(); }
 }
 
 type AnimationConfig = {

Libraries/Animated/src/__tests__/Animated-test.js

@@ -142,6 +142,9 @@ describe('Animated', () => {
     expect(anim.__getValue()).toBe(15);
   });
 
+  it('convert to JSON', () => {
+    expect(JSON.stringify(new Animated.Value(10))).toBe('10');
+  });
 });