Animated.multiply and Animated.add to combine animated values

Summary:
This PR was created in response to feedback from an older PR: https://github.com/facebook/react-native/pull/2045

The `.interpolate()` API of Animated values is quite effective to accomplish transformations based on a single animated value, but currently there is a class of animations that is impossible: animations being driven by more than one value.

Usage would be like the following:

```js
getInitialState: function() {
  return {
    panY: new Animated.Value(0),
    offset: new Animated.Value(0),
  };
}
```

```js
var scale = Animated.add(panY, offset).interpolate({
  inputRange: [...],
  outputRange: [...],
});
```

I have a real use case for this, and I cannot think of any way to accomplish what I need without an API like this.

The animation I am trying to accomplish is I have a PanResponder being used to detect both x and y panning. The y-axis panning drives a 3d sroll-like animation (which we can call `panY`), and the x-axis panning is driving a "swipe-to-remove" animation (
Closes https://github.com/facebook/react-native/pull/4395

Reviewed By: svcscm

Differential Revision: D2731305

Pulled By: vjeux

fb-gh-sync-id: 3b9422f10c7e7f3b3ecd270aeed8ea92315a89e9

Libraries/Animated/src/AnimatedImplementation.js

@@ -840,6 +840,64 @@ class AnimatedInterpolation extends AnimatedWithChildren {
   }
 }
 
+class AnimatedAddition extends AnimatedWithChildren {
+  _a: Animated;
+  _b: Animated;
+
+  constructor(a: Animated, b: Animated) {
+    super();
+    this._a = a;
+    this._b = b;
+  }
+
+  __getValue(): number {
+    return this._a.__getValue() + this._b.__getValue();
+  }
+
+  interpolate(config: InterpolationConfigType): AnimatedInterpolation {
+    return new AnimatedInterpolation(this, Interpolation.create(config));
+  }
+
+  __attach(): void {
+    this._a.__addChild(this);
+    this._b.__addChild(this);
+  }
+
+  __detach(): void {
+    this._a.__removeChild(this);
+    this._b.__removeChild(this);
+  }
+}
+
+class AnimatedMultiplication extends AnimatedWithChildren {
+  _a: Animated;
+  _b: Animated;
+
+  constructor(a: Animated, b: Animated) {
+    super();
+    this._a = a;
+    this._b = b;
+  }
+
+  __getValue(): number {
+    return this._a.__getValue() * this._b.__getValue();
+  }
+
+  interpolate(config: InterpolationConfigType): AnimatedInterpolation {
+    return new AnimatedInterpolation(this, Interpolation.create(config));
+  }
+
+  __attach(): void {
+    this._a.__addChild(this);
+    this._b.__addChild(this);
+  }
+
+  __detach(): void {
+    this._a.__removeChild(this);
+    this._b.__removeChild(this);
+  }
+}
+
 class AnimatedTransform extends AnimatedWithChildren {
   _transforms: Array<Object>;
 
@@ -1161,6 +1219,20 @@ type CompositeAnimation = {
   stop: () => void;
 };
 
+var add = function(
+  a: Animated,
+  b: Animated
+): AnimatedAddition {
+  return new AnimatedAddition(a, b);
+};
+
+var multiply = function(
+  a: Animated,
+  b: Animated
+): AnimatedMultiplication {
+  return new AnimatedMultiplication(a, b);
+};
+
 var maybeVectorAnim = function(
   value: AnimatedValue | AnimatedValueXY,
   config: Object,
@@ -1521,6 +1593,17 @@ module.exports = {
    */
   spring,
 
+  /**
+   * Creates a new Animated value composed from two Animated values added
+   * together.
+   */
+  add,
+  /**
+   * Creates a new Animated value composed from two Animated values multiplied
+   * together.
+   */
+  multiply,
+
   /**
    * Starts an animation after the given delay.
    */