Make sure that AnimatedTracking is started when CompositeAnimation.st…

…art is called

Summary:
In Animated, when a toValue of AnimatedValue (as opposed to a number) is passed in, the [AnimatedValue starts tracking via AnimatedTracking](https://www.internalfb.com/code/fbsource/[b688f3747a706236fce300636978ed1ca5e4081a]/xplat/js/react-native-github/Libraries/Animated/AnimatedImplementation.js?lines=142) but it doesn't actually start animating even if start() is called on the animation.

This behavior is inconsistent with a toValue of a number, which executes immediately on start(). This diff adds a call to AnimatedTracking.update within AnimatedValue.track, which starts the tracking animation.

Changelog:
[General][Fixed] - Fixes execution of animation when a toValue of AnimatedValue is used.

Reviewed By: JoshuaGross

Differential Revision: D33800373

fbshipit-source-id: 85ee6f51bc2bb2e078b586b076a8d1dfe92c1155

Libraries/Animated/__tests__/Animated-test.js

@@ -782,6 +782,19 @@ describe('Animated tests', () => {
       value1.setValue(1492);
       expect(value2.__getValue()).toBe(7);
     });
+
+    it('should start tracking immediately on animation start', () => {
+      const value1 = new Animated.Value(42);
+      const value2 = new Animated.Value(0);
+      Animated.timing(value2, {
+        toValue: value1,
+        duration: 0,
+        useNativeDriver: false,
+      }).start();
+      expect(value2.__getValue()).toBe(42);
+      value1.setValue(7);
+      expect(value2.__getValue()).toBe(7);
+    });
   });
 
   describe('Animated Vectors', () => {

Libraries/Animated/nodes/AnimatedValue.js

@@ -270,6 +270,8 @@ class AnimatedValue extends AnimatedWithChildren {
   track(tracking: AnimatedTracking): void {
     this.stopTracking();
     this._tracking = tracking;
+    // Make sure that the tracking animation starts executing
+    this._tracking && this._tracking.update();
   }
 
   _updateValue(value: number, flush: boolean): void {