fix(iOS): Change retrieving window in FullWindowOverlay

[tboba]

Description

This PR is a follow up for the PR #1526.

Currently, in React Native there's RCTKeyWindow() method which goes through each window of RCTSharedApplication() and selects the window that is a key window of a given window. This is a better solution, since we don't rely on a single keyWindow from RCTSharedApplication. I've also added a support for Fabric.

I've checked how does FullWindowOverlay work with the change and it works as usual. Closes #1526.

Changes

• Changed the method of retrieving window from delegate.window to RCTKeyWindow()

Test code and steps to reproduce

You can run Test1844 and check if FullWindowOverlay works correctly.

Checklist

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