Add Animated.ValueXY.extractOffset

Summary:
The `extractOffset` method is previously implemented on `Animated.Value`, along with `setOffset` and `flattenOffset`. Both of the latter methods are proxied on `Animated.ValueXY`, but `extractOffset` is not. This commit adds the missing method.

When creating draggable elements in XY space, in order to compensate for previous gestures' offsets, we currently need to do something like the following at the beginning or end of each gesture:
```
xy.setOffset(xy.__getValue());
xy.setValue({x: 0, y: 0});
```

After this commit, the API is equivalent to using a plain `Animated.Value`:
```
xy.extractOffset();
```
Closes facebook#12193

Differential Revision: D4508717

Pulled By: mkonicek

fbshipit-source-id: 0976cf1ab68538e59023ffaa2539eb62779ad874

Libraries/Animated/src/AnimatedImplementation.js

@@ -999,6 +999,11 @@ class AnimatedValueXY extends AnimatedWithChildren {
     this.y.flattenOffset();
   }
 
+  extractOffset(): void {
+    this.x.extractOffset();
+    this.y.extractOffset();
+  }
+
   __getValue(): {x: number, y: number} {
     return {
       x: this.x.__getValue(),