Fixup contentLength invalidation logic (#38733)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/38733

I was working under the assumption that Fabric fired layout events bottom up, but it actually fires them top-down, in constrast to Paper.

Previous invalidation logic wasn't quite correct when fired bottom-up. This corrects the logic by:
1. Deriving direction based on initial event ordering
2. Use last cached contentLength if we are on Fabric (top-down)
3. Use future contentLength if we are on Paper (bottom-up)

Changelog:
[General][Fixed] - Fixup contentLength invalidation logic

Reviewed By: rozele

Differential Revision: D47978638

fbshipit-source-id: 3446d08aa34397b4e6bd9924dad0eba36a12a115