Breakup XamlUIService into separate APIs

Summary

It feels like over time, XamlUIService has become a bit of a dumping ground for various APIs.

1. It was originally created in https://github.com/microsoft/react-native-windows/commit/2809c9a14a68cdbaa4b735b189197b5692af0d53 to handle looking up tags from elements.
2. Then we added an option to get a singleton XamlRoot in https://github.com/microsoft/react-native-windows/commit/f52d4942a42ed75d6fa92594897d2f3d7db1b7d7
3. Then we added options to set the HWND in https://github.com/microsoft/react-native-windows/commit/bdd23dc452f4167769e3166513524f263fd3cda0
4. The an accessibility workaround in https://github.com/microsoft/react-native-windows/commit/2dfb964e57020226828867f816bd31cff25814b8
5. There is a proposed addition for getting the React tag and ReactRootView here: https://github.com/microsoft/react-native-windows/pull/10403
6. And another proposal for adding a mechanism to invoke nested Yoga layout here: https://github.com/microsoft/react-native-windows/pull/10237

It feels like we should separate concerns a bit here and expose a few new APIs:

1. XamlUIService can remain as a translation layer between XAML views and the React shadow tree to fill the gap of missing shadow nodes in the exposed ABI, e.g.:

void XamlUIService::GetElementFromTag(int64_t tag)
void XamlUIService::GetReactTag(xaml::DependencyObject view)
ReactRootView XamlUIService::GetReactRootView(xaml::DependencyObject view)

2. A per root view ReactWindowService could be added to ReactRootView to handle things like getting / setting the HWND, getting / setting the XamlRoot, getting / setting the accessibility root, etc.

ReactWindowService ReactRootView::WindowService()
void ReactWindowService::XamlRoot(XamlRoot root);
XamlRoot ReactWindowService::XamlRoot();
void ReactWindowService::HWND(int64_t);
uint64_t ReactWindowService::HWND();
// Do not exist yet, but would be needed to feed scale factors to Yoga
double ReactWindowService::ScaleFactor();
void ReactWindowService::ScaleFactor(double scaleFactor);

3. A service for the AccessibilityInfoModule, AccessibilityInfoService:

AccessibilityInfoService ReactContext::AccessibilityInfoService();
void AccessibilityInfoService::AccessibleRoot(xaml::FrameworkElement root);
xaml::FrameworkElement AccessibilityInfoService::AccessibleRoot();

4. An API for event dispatching, EventDispatcher:

EventDispatcher ReactContext::EventDispatcher()
void EventDispatcher::DispatchEvent(...)
// Does not exist yet, but would be useful
void EventDispatcher::DispatchCoalescedEvent(...)

5. An API for UIManager, UIManagerService:

UIManagerService ReactContext::UIManagerService()
void UIManagerService::UpdateYogaLayout(int64_t tag);
// Does not exist yet, but would expose public surfaces for functionality in iOS/Android
void UIManagerService::AddOnBatchCompleteListener(...);

Motivation

Separation of concerns for the collection of methods exposed in XamlUIService.

Basic Example

No response

Open Questions

No response

Start by reading the existing XamlUIService and the linked commits and proposals for ReactRootView, nested Yoga layout, and related APIs. Done means agreeing on service boundaries and migrating the relevant responsibilities without leaving XamlUIService as a dumping ground; the issue names no files or tests.