Fix modal and alert dismissal bugs

[sryze]

Fixes #22237

Summary:

1. Fixed a bug with dismissing modals that caused the modal to stay on screen after being dismissed if some other view controller is presented on top of it.

2. Fixed an issue where dismissing a modal (by changing its visible property) would also hide alerts presented on top of it (i.e. shown after the modal became visible). This is achieved by presenting each alert in a separate UIWindow instance.

3. Fixed a bug where showing two modals rapidly and hiding them would leave an empty (invisible) RCTModalHostViewController hanging on screen, which made it impossible to interact with the app as the view controller is display on top of everything.

These changes affect iOS only (no JS/Android changes).

Test Plan:

Need some help with filling this section...

Changelog:

[iOS] [Fixed] - Fixed various issues with dismissing alerts and modals

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[sryze]

@shergin, can you please review/merge this pull request?

BTW I don't think the failure of "test_objc" is related to my changes in this PR.

[sryze]

After more testing I discovered that [3] is not fully fixed as I ran into the same bug again today. I'll try to make a repro later.

While debugging this I found that after the call todismissViewController:animated: the modal view controller is not deallocated because the main (root) view controller still holds a strong reference to it... I'm not sure if it's a React Native bug or iOS bug. For some reason it only happens in release builds.

[prateekgoel]

Any progress on this?

[shergin]

This diff contains three things (and this complicates review process):

• Alert-related fixes. Should we decouple this component from the core? See the comment in the code.
• Modal-related fixes.
• Code-style fixes. Feel free to publish them separately.

Could you please to split this up?

[shergin]

Retaining anything in ViewManager is very bad idea in general (and especially for UIKit things).

[cpojer]

As @shergin pointed out this PR should be split into three separate PRs. Would you mind opening those? I'm going to close this one because it's too big to be merged as one. Ideally every PR only has one main objective (one bugfix, one new feature, etc.).