Disable shadow tree introspection by default (#43578)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/43578

Changelog: [internal]

We have a feature to do some validation of the mount operations when committing new trees in Fabric. That's very slow but it was ok before because we were only doing this in debug mode and in the JS thread. We're moving some of this work to the UI thread instead and we're seeing an impact on scroll performance.

This disables this feature by default but leaves it in code to enable it when necessary for debugging.

Reviewed By: NickGerleman, sammy-SC

Differential Revision: D55138795

fbshipit-source-id: 45ca47ae2562cecb070691bf33d95c9108a9eca3