Provide a way for iOS plugins to know what window/view to interact with

[stuartmorgan]

Several of our 1P plugins present view controllers over the Flutter view; GIDSignIn needs this information, for example. Currently, we do this using UIApplication.sharedApplication.keyWindow, but in iOS 13 keyWindow is deprecated because of the introduction of scenes on iPads.

To make Flutter scene-friendly, we should provide an alternate way of getting that information. On the macOS embedding, for instance, I added a way to get the NSView of the Flutter content via the plugin registrar, which allows plugins to get the window they are part of for things like showing sheets. (I keep finding 3P plugins that ignore that and use the key window instead, but 🤷🏻 ). We should consider doing the same for iOS, or providing some alternate solution.

/cc @jmagman

[stuartmorgan]

I'm going to temporarily suppress these warnings with a TODO referencing this issue, so once we have a solution we should look for this issue number in flutter/plugins and update the relevant code.

[stuartmorgan]

On the macOS embedding, for instance, I added a way to get the NSView of the Flutter content via the plugin registrar

This API is being adjusted as part of multi-window support; we should take those adjustments into account when designing a corresponding iOS API.

[cyanglaz]

On the macOS embedding, for instance, I added a way to get the NSView of the Flutter content via the plugin registrar, which allows plugins to get the window they are part of for things like showing sheets.

This is the API on macOS: https://github.com/flutter/engine/blame/main/shell/platform/darwin/macos/framework/Headers/FlutterPluginRegistrarMacOS.h#L45

Multi-view version:
https://github.com/flutter/engine/blame/main/shell/platform/darwin/macos/framework/Headers/FlutterPluginRegistrarMacOS.h#L50