Prepend the accessibility placeholder (#51050)

Prepends the accesibility placeholder. Previously it was being appended (last child) to the body. 

Relevant Issues are:

* Design doc: https://flutter.dev/go/focus-management 
* Focus in web multiview: flutter/flutter#137443
* Platform dispatcher changes: #49841

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

Author: tugorez

lib/web_ui/lib/src/engine/platform_dispatcher.dart

@@ -80,7 +80,7 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher {
     registerHotRestartListener(dispose);
     AppLifecycleState.instance.addListener(_setAppLifecycleState);
     ViewFocusBinding.instance.addListener(invokeOnViewFocusChange);
-    domDocument.body?.append(accessibilityPlaceholder);
+    domDocument.body?.prepend(accessibilityPlaceholder);
     _onViewDisposedListener = viewManager.onViewDisposed.listen((_) {
       // Send a metrics changed event to the framework when a view is disposed.
       // View creation/resize is handled by the `_didResize` handler in the

lib/web_ui/test/engine/platform_dispatcher/platform_dispatcher_test.dart

@@ -409,8 +409,9 @@ void testMain() {
       });
     });
 
-    test('appends an accesibility placeholder', () {
+    test('adds the accesibility placeholder', () {
       expect(dispatcher.accessibilityPlaceholder.isConnected, isTrue);
+      expect(domDocument.body!.children.first, dispatcher.accessibilityPlaceholder);
     });
 
     test('removes the accesibility placeholder', () {