Backport #19623 (overriding WebView/UrlStore) to 6.5

[tmeasday]

Is your feature request related to a problem? Please describe

React Native wants to be able to define a non-web version of the view+store without needing to monkey patch and polyfill.

We have a solution in #19623 but we need to backport it to 6.5

Describe the solution you'd like

As #19623 but leaving out the complex typing changes.

[yannbf]

Quick reminder to also change code in the instrumenter if you're backporting this to 6.5, else addon-interactions will break as it refers to urlStore to get the story ids.

[tmeasday]

This got sorted out in #19975 -- the change was really simple in 6.x so the above won't be an issue. Thanks for highlighting @yannbf