VirtualizedList: Fix spacer size calculation (#18105)

Summary:
See https://github.com/facebook/react-native/issues/18104

Tested the above snack with:
- stickyHeadersEnabled true/false
- initialScrollIndex set and not

Visibly verified consistent rendering.

[GENERAL] [BUGFIX] [VirtualizedList] - Fix for jumpy content when initialScrollIndex specified
Pull Request resolved: https://github.com/facebook/react-native/pull/18105

Differential Revision: D8382122

Pulled By: sahrens

fbshipit-source-id: 9421351469e8684bc61438605abbd9988b664c29

Libraries/Lists/VirtualizedList.js

@@ -805,8 +805,7 @@ class VirtualizedList extends React.PureComponent<Props, State> {
             if (stickyIndicesFromProps.has(ii + stickyOffset)) {
               const initBlock = this._getFrameMetricsApprox(lastInitialIndex);
               const stickyBlock = this._getFrameMetricsApprox(ii);
-              const leadSpace =
-                stickyBlock.offset - (initBlock.offset + initBlock.length);
+              const leadSpace = stickyBlock.offset - initBlock.offset;
               cells.push(
                 <View key="$sticky_lead" style={{[spacerKey]: leadSpace}} />,
               );