Fix inverted `contentOffset` in scroll events in RTL (#39031)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/39031

`UIScrollView` `contentOffset` is flow-relative, so `x` is relative to the right edge of the screen. This coordinate space disagrees with layout events, `scrollTo` coordinates, and other platforms.

This applies the same logic we use for inverting `scrollTo` coordinates to invert `contentOffset` in scroll events, in both Paper and Fabric. We then remove the iOS specific workaround we have in VirtualizedList.

I did not test `contentInset` as part of this, whose structure has explicit edges like `left` and `right`.

Changelog:
[iOS][Fixed] - Fix inverted `contentOffset` in scroll events in RTL

Reviewed By: rozele

Differential Revision: D48379915

fbshipit-source-id: 8a9cbb01608e79ef3b179a76fbe3997a0cd23845