added onComplete callback to animation config

Summary:
This fix provides possibility to subscribe to a child animation lifecycle. You'll be able to observe every single animation:
```
Animated.sequence([
            Animated.timing(
                this.state.scale,
                {
                    toValue: 0,
                    duration: 300,
                    onComplete: () => this.setState({someProp: 'new value'})
                }
            ),
            Animated.timing(
                this.state.scale,
                {
                    toValue: 1,
                    duration: 300
                }
            ),
        ]).start();
```
`state.someProp`, will updated with `'new value'` when the first animation will be completed.
Closes https://github.com/facebook/react-native/pull/8494

Reviewed By: javache

Differential Revision: D3735322

Pulled By: foghina

fbshipit-source-id: fb69a4b993f7ab6a16da4fdd670e6c0b11c93517

Libraries/Animated/src/AnimatedImplementation.js

@@ -74,6 +74,7 @@ class Animated {
 type AnimationConfig = {
   isInteraction?: bool,
   useNativeDriver?: bool,
+  onComplete?: ?EndCallback,
 };
 
 // Important note: start() and stop() will only be called at most once.
@@ -1676,6 +1677,16 @@ var modulo = function(
   return new AnimatedModulo(a, modulus);
 };
 
+const _combineCallbacks = function(callback: ?EndCallback, config : AnimationConfig) {
+  if (callback && config.onComplete) {
+    return (...args) => {
+      config.onComplete && config.onComplete(...args);
+      callback && callback(...args);
+    };
+  } else {
+    return callback || config.onComplete;
+  }
+};
 
 var maybeVectorAnim = function(
   value: AnimatedValue | AnimatedValueXY,
@@ -1707,6 +1718,7 @@ var spring = function(
 ): CompositeAnimation {
   return maybeVectorAnim(value, config, spring) || {
     start: function(callback?: ?EndCallback): void {
+      callback = _combineCallbacks(callback, config);
       var singleValue: any = value;
       var singleConfig: any = config;
       singleValue.stopTracking();
@@ -1735,6 +1747,7 @@ var timing = function(
 ): CompositeAnimation {
   return maybeVectorAnim(value, config, timing) || {
     start: function(callback?: ?EndCallback): void {
+      callback = _combineCallbacks(callback, config);
       var singleValue: any = value;
       var singleConfig: any = config;
       singleValue.stopTracking();
@@ -1763,6 +1776,7 @@ var decay = function(
 ): CompositeAnimation {
   return maybeVectorAnim(value, config, decay) || {
     start: function(callback?: ?EndCallback): void {
+      callback = _combineCallbacks(callback, config);
       var singleValue: any = value;
       var singleConfig: any = config;
       singleValue.stopTracking();