Fix detection of Fabric in ScrollViewStickyHeader

Summary:
Changelog: [internal]

Revert a change introduced in #30333 where rickhanlonii asked it to be reverted as well.

The change breaks sticky header in Fabric.

Reviewed By: rubennorte

Differential Revision: D25883861

fbshipit-source-id: b01305c6def390d0664c7be939ab3fc72186a07a

Libraries/Components/ScrollView/ScrollViewStickyHeader.js

@@ -175,9 +175,10 @@ class ScrollViewStickyHeader extends React.Component<Props, State> {
 
   render(): React.Node {
     // Fabric Detection
-    // eslint-disable-next-line dot-notation
     const isFabric = !!(
-      this._ref && this._ref._internalInstanceHandle?.stateNode?.canonical
+      // An internal transform mangles variables with leading "_" as private.
+      // eslint-disable-next-line dot-notation
+      (this._ref && this._ref['_internalInstanceHandle']?.stateNode?.canonical)
     );
 
     // Initially and in the case of updated props or layout, we