refactor(iOS): limit scope of shouldUseCustomBackBarButtonItem flag

[maciekstosio]

Description / Changes

This is continuation of #2860 that proposes small refactor around back button logic. This area is very confusing and in my opinion this is due to flag shouldUseCustomBackBarButtonItem that breaks the flow of other if's and causes other props to not work. Not we contained this flag in if (visible) which currently is the only case when backButtonItem might be used after the changes from #2800.

Test code and steps to reproduce

see FabricExample/e2e/issuesTests/Test2809.e2e.ts and apps/src/tests/Test2809/index.tsx

Checklist

• Ensured that CI passes

[kkafar]

I don't think this works.

Consider scenario: config.disableBackButtonMenu == true && config.isBackTitleVisible == false

You won't use custom back button then -> you will break the behaviour of disableBackButtonMenu prop. We can't merge this.

[maciekstosio]

But it didn't work before these changes too. When backTitleVisible == false, then in else statement we set shouldUseCustomBackBarButtonItem = NO and backButtonItem won't be set, thus we use default UIBarButtonItem implementation with backButtonMenu

As far as I remember this is what we decided in #2800, the backTitleVisible works as a kill switch.

[kkafar]

I think we're good. Thanks!