fix(aio): do not show new document until embedded components are ready (#18428)

Previously, the document was shown as soon as the HTML was received, but before
the embedded components were ready (e.g. downloaded and instantiated). This
caused FOUC (Flash Of Uninstantiated Components).
This commit fixes it by preparing the new document in an off-DOM node and
swapping the nodes when the embedded components are ready.

PR Close #18428

aio/src/testing/doc-viewer-utils.ts

@@ -18,10 +18,13 @@ import { MockLogger } from 'testing/logger.service';
 
 export class TestDocViewerComponent extends DocViewerComponent {
   embeddedComponentRefs: ComponentRef<any>[];
+  currViewContainer: HTMLElement;
+  nextViewContainer: HTMLElement;
 
   addTitleAndToc(docId: string): void { return null as any; }
   destroyEmbeddedComponents(): void { return null as any; }
   render(doc: DocumentContents): Observable<void> { return null as any; }
+  swapViews(): Observable<void> { return null as any; }
 }