Summary:This change adds suport native animated support for Animated.add.
Animated.add lets you declare node that outputs a sum of it input nodes.
**Test Plan**
Play with the following playground app: https://gist.github.com/39de37faf07480fcd7d1
Run JS tests: `npm test Libraries/Animated/src/__tests__/AnimatedNative-test.js`
Run java tests: `buck test ReactAndroid/src/test/java/com/facebook/react/animated`
Closes https://github.com/facebook/react-native/pull/6641
Differential Revision: D3195963
fb-gh-sync-id: bb1e1a36821a0e071ad0e7d0fa99ce0d6b088b0a
fbshipit-source-id: bb1e1a36821a0e071ad0e7d0fa99ce0d6b088b0a
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