Restore macOS <Image> and ActionSheetIOS. Cherry pick multi windows UIScene support from upstream.

[tom-un]

Please select one of the following

• I am removing an existing difference between facebook/react-native and microsoft/react-native 👍
• I am cherry-picking a change from Facebook's react-native into microsoft/react-native 👍
• I am making a fix / change for the macOS implementation of react-native
• I am making a change required for Microsoft usage of react-native

Since the last merge from upstream to the fork two manual merge errors impacted macOS:

• In RCTImageView.m the macOS version of the _imageView ivar was not being initialized breaking <Image> components.
• In react-native-implementation.macos.js the ActionSheetIOS component was not being exposed. Although named ActionSheetIOS in the fork it creates NSMenu popups for macOS and UIAlertControllers for iOS.

Since iOS 13 the RCTKeyWindow() function is broken in apps that use UIScene's for multi window support. It also breaks RCTRedBox. If an app that uses UIScene's (like Office apps) show an RCTRedBox error message, the red box window is invisible. Then if a ActionSheetIOS presents a UIAlertController it is presented on the invisible Red Box windows making the action sheet or alert invisible. Cherry pick the upstream fix facebook#28147. Future merges into this fork from upstream will get the more comprehensive UIScene support that started with facebook#28058.

Focus areas to test

The Image test in RNTester was used to verify that components is restored. UIScene support was tested by manually integrating this branch into Office.

Microsoft Reviewers: Open in CodeFlow