Fix maintainVisibleContentPosition with VirtualizedList

[janicduplessis]

#8 targeting 0.69-stable branch.

[janicduplessis]

There might be a conflict here when merging onStartReached change

[roryabraham]

If we want to make this PR separate from the onStartReached prop implementation, then I think it might be pretty valuable to have reproduction steps that aren't dependent upon onStartReached?

My concern with separating them is that the onStartReached prop implementation on its own doesn't really work well without these changes, because 90% of the time people wanting to use onStartReached will be using VirtualizedList components instead of ScrollView.

[janicduplessis]

I think we can work on getting onStartReached merged first and then this one, we can get the repro example in later (maybe make it a separate one).

The main reason for splitting it is to not have this code also in the onStartReached PR as it would make it a lot bigger and harder to get merged in.