[Animated] Send a final update with toValue for spring

Summary:
Animated.spring is not guarantee to stabilize at exactly toValue (determined by restDisplacementThreshold). It is a bit annoying that the last value is not toValue, it makes the logs harder to read and also prevents you from writing code like value === toValue. Instead you need to track it down somewhere else.

Libraries/Animation/Animated/Animated.js

@@ -479,7 +479,13 @@ class SpringAnimation extends Animation {
     if (this._tension !== 0) {
       isDisplacement = Math.abs(this._toValue - position) <= this._restDisplacementThreshold;
     }
+
     if (isOvershooting || (isVelocity && isDisplacement)) {
+      if (this._tension !== 0) {
+        // Ensure that we end up with a round value
+        this._onUpdate(this._toValue);
+      }
+
       this.__debouncedOnEnd({finished: true});
       return;
     }

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

@@ -127,6 +127,19 @@ describe('Animated', () => {
     Animated.spring(anim, {toValue: 0, velocity: 0}).start(callback);
     expect(callback).toBeCalled();
   });
+
+  it('send toValue when a spring stops', () => {
+    var anim = new Animated.Value(0);
+    var listener = jest.genMockFunction();
+    anim.addListener(listener);
+    Animated.spring(anim, {toValue: 15}).start();
+    jest.runAllTimers();
+    var lastValue = listener.mock.calls[listener.mock.calls.length - 2][0].value;
+    expect(lastValue).not.toBe(15);
+    expect(lastValue).toBeCloseTo(15);
+    expect(anim.__getValue()).toBe(15);
+  });
+
 });