Fix sticky header not sticking on first render in ScrollView

Summary:
# The bug
Sticky headers would not "stick" to the top of the ScrollView on initial render. On subsequent redners, all sticking would work correctly.

# Why the bug existed
This code to initialize the animated values used for sticky headers was in `UNSAFE_componentWillMount` prior to D26375818 (1641d46). `UNSAFE_componentWillMount` is called before `render`.

In D26375818 (1641d46), I moved the code into `componentDidMount`, which is called after `render`.

This caused a problem because code in `render` was relying on these initializations being done already.

# How I resolved the bug
To resolve this, I initialize these values in the constructor.

# Reference
Docs for React mount ordering: https://reactjs.org/docs/react-component.html#mounting

Changelog:
[General][Fixed] Fix sticky header not sticking on first render in ScrollView

Reviewed By: nadiia

Differential Revision: D26792003

fbshipit-source-id: c575e8cdd1d986ce3c38941d95d763e329e74874

Libraries/Components/ScrollView/ScrollView.js

@@ -719,6 +719,13 @@ class ScrollView extends React.Component<Props, State> {
   static Context: typeof ScrollViewContext = ScrollViewContext;
   constructor(props: Props) {
     super(props);
+
+    this._scrollAnimatedValue = new AnimatedImplementation.Value(
+      this.props.contentOffset?.y ?? 0,
+    );
+    this._scrollAnimatedValue.setOffset(this.props.contentInset?.top ?? 0);
+    this._stickyHeaderRefs = new Map();
+    this._headerLayoutYs = new Map();
   }
 
   _scrollAnimatedValue: AnimatedImplementation.Value = new AnimatedImplementation.Value(
@@ -789,13 +796,6 @@ class ScrollView extends React.Component<Props, State> {
       this.scrollResponderKeyboardDidHide,
     );
 
-    this._scrollAnimatedValue = new AnimatedImplementation.Value(
-      this.props.contentOffset?.y ?? 0,
-    );
-    this._scrollAnimatedValue.setOffset(this.props.contentInset?.top ?? 0);
-    this._stickyHeaderRefs = new Map();
-    this._headerLayoutYs = new Map();
-
     this._updateAnimatedNodeAttachment();
   }