[Windows] Allow view controllers to not own the engine

[loic-sharma]

Adds FlutterDesktopEngineCreateViewController to the Windows embedder's C API. Example usage:

// Create the engine.
FlutterDesktopEngineProperties engine_properties = {};
auto engine = FlutterDesktopEngineCreate(&engine_properties);

// Create a views controller.
FlutterDesktopViewControllerProperties properties = {};
properties.width = 400;
properties.height = 400;

auto controller = FlutterDesktopEngineCreateViewController(engine, &properties);

// Run app...

// Destroy the view controller. This does not destroy the engine. 
FlutterDesktopViewControllerDestroy(controller);

// Destroy the engine.
FlutterDesktopEngineDestroy(engine);

This new API creates a view controller that does not own the engine, unlike the existing FlutterDesktopEngineCreateViewController. In the future, this new API will allow a Windows app to have multiple views that share a single engine.

This API is in the flutter_windows_internal.h, making it "pubternal". This API is not in the public Windows header files, however, one can link against this API for testing purposes. This API will be promoted to the public Windows header files once the Windows embedder is ready to have multiple views.

Relevant design documents:

1. https://flutter.dev/go/desktop-multi-view-runner-apis
2. http://flutter.dev/go/windows-multi-view-ownership-updates

Relevant issues:

1. [Windows] Add multi-view runner APIs flutter#143767
2. ☂️ Multi View for Windows/MacOS flutter#142845

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[dkwingsmt]

LGTM

[dkwingsmt]

We might want to add a similar paragraph to FlutterDesktopViewControllerCreate as well, so that they can refer to each other. And also we might want to mention that:

// In an app with multiple view controllers, it is highly recommend to use
// `FlutterDesktopEngineCreateViewController` for all view controllers and avoid
// `FlutterDesktopViewControllerCreate`.

[loic-sharma]

We'll definitely want a blurb like this eventually, however, FlutterDesktopEngineCreateViewController isn't in the public header yet. Adding this right now would be confusing to our customers as they can't use this API yet.

I'll make sure to add such a comment when we make this runner API public.

[dkwingsmt]

Add

// If `engine_` is not null, it will be deallocated upon
// the view controller's destruction.

[loic-sharma]

I tweaked the wording, hopefully it's clearer now.