Update _scrollAnimatedValue offset of ScrollViews. (#19481)
Update _scrollAnimatedValue offset of ScrollViews. (#19481)

Summary:
`_scrollAnimatedValue` offset of ScrollView is set once in `UNSAFE_componentWillMount` but it is never updated.
It causes unexpected render result.

![rn-scrollview-fix1](https://user-images.githubusercontent.com/143255/40640292-61843eca-6350-11e8-9412-f5383ea65ea0.gif)

So I suggest to update `_scrollAnimatedValue` offset when ScrollView contentInset is updated.
Pull Request resolved: https://github.com/facebook/react-native/pull/19481

Differential Revision: D14223304

Pulled By: cpojer

fbshipit-source-id: 4191cfcf6414adf3a0abd156517d5f9778565671

facebook/react-native,hoangpham95/react-native,hoangpham95/react-native,myntra/react-native,hammerandchisel/react-native,myntra/react-native,hoangpham95/react-native,arthuralee/react-native,exponent/react-native,pandiaraj44/react-native,arthuralee/react-native,facebook/react-native,pandiaraj44/react-native,pandiaraj44/react-native,arthuralee/react-native,javache/react-native,hoangpham95/react-native,exponentjs/react-native,myntra/react-native,arthuralee/react-native,hammerandchisel/react-native,hammerandchisel/react-native,hammerandchisel/react-native,javache/react-native,exponent/react-native,javache/react-native,janicduplessis/react-native,exponentjs/react-native,pandiaraj44/react-native,janicduplessis/react-native,exponent/react-native,facebook/react-native,myntra/react-native,facebook/react-native,exponent/react-native,pandiaraj44/react-native,javache/react-native,exponent/react-native,javache/react-native,facebook/react-native,hammerandchisel/react-native,myntra/react-native,exponent/react-native,janicduplessis/react-native,myntra/react-native,myntra/react-native,javache/react-native,exponentjs/react-native,arthuralee/react-native,exponent/react-native,exponentjs/react-native,facebook/react-native,exponentjs/react-native,hoangpham95/react-native,janicduplessis/react-native,hoangpham95/react-native,hoangpham95/react-native,hammerandchisel/react-native,facebook/react-native,pandiaraj44/react-native,exponentjs/react-native,exponentjs/react-native,hammerandchisel/react-native,myntra/react-native,exponent/react-native,facebook/react-native,exponentjs/react-native,facebook/react-native,myntra/react-native,janicduplessis/react-native,janicduplessis/react-native,javache/react-native,janicduplessis/react-native,javache/react-native,hammerandchisel/react-native,janicduplessis/react-native,javache/react-native,pandiaraj44/react-native,pandiaraj44/react-native,hoangpham95/react-native