fix(iOS, Paper): prevent double modal dismissal

[kkafar]

Description

Fixes #2525

View hierarchy for modal with header looks kinda like this:

UITransitionView -> Screen -> Stack (UINavigationController) -> Screen -> content

calling [_controller dismissViewControllerAnimated:YES completion:nil] on the Stack ☝️, according to documentation
should dismiss the view controller itself and any view controllers presented from that view controller. However in practive it dismisses not only itself and "above" modal, but also a single modal "below". I'm not sure
why it is the case. Removing this line of code introduces a bug: during development, when multiple modals are opened and you reload the react-native, the modals could be left in stuck state. I'll proceed, as this bug
bug in development is of much lesser severity.

I've decided to move the call "one up" the view controller hierarchy, to avoid calling dismiss directly on UINavigationController as it looks like it causes the problem. This fixes the problem & keeps the development behaviour working as intended.

The bug was introduced with

• fix(iOS,Paper): fix broken modal go-back animation #2496

On Fabric this is not an issue, because invalidate is not called (Fabric does not support RCTInvalidating protocol)

Changes

Described above ☝️

Test code and steps to reproduce

I've enhanced TestModalNavigation test case to cover this issue.

Checklist

• Included code example that can be used to test this change
• Ensured that CI passes