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: #19481

Differential Revision: D14223304

Pulled By: cpojer

fbshipit-source-id: 4191cfcf6414adf3a0abd156517d5f9778565671

Libraries/Components/ScrollView/ScrollView.js

@@ -658,6 +658,18 @@ class ScrollView extends React.Component<Props, State> {
     this._headerLayoutYs = new Map();
   }
 
+  UNSAFE_componentWillReceiveProps(nextProps: Props) {
+    const currentContentInsetTop = this.props.contentInset
+      ? this.props.contentInset.top
+      : 0;
+    const nextContentInsetTop = nextProps.contentInset
+      ? nextProps.contentInset.top
+      : 0;
+    if (currentContentInsetTop !== nextContentInsetTop) {
+      this._scrollAnimatedValue.setOffset(nextContentInsetTop || 0);
+    }
+  }
+
   componentDidMount() {
     this._updateAnimatedNodeAttachment();
   }