fix(aio): activate ServiceWorker updates asap (#17699)

Previouly, whenever a new ServiceWorker update was detected the user was
prompted to update (with a notification). This turned out to be more distracting
than helpful. Also, one would get notifications on all open browser tabs/windows
and had to manually reload each one in order for the whole content (including
the app) to be updated.

This commit changes the update strategy as follows:
- Whenever a new update is detected, it is immediately activated (and all
  tabs/windows will be notified).
- Once an update is activated (regardless of whether the activation was
  initiated by the current tab/window or not), a flag will be set to do a
  "full page navigation" the next time the user navigates to a document.

Benefits:
- All tabs/windows are updated asap.
- The updates are applied authomatically, without the user's needing to do
  anything.
- The updates are applied in a way that:
  a. Ensures that the app and content versions are always compatible.
  b. Does not distract the user from their usual workflow.

NOTE:
The "full page navigation" may cause a flash (while the page is loading from
scratch), but this is expected to be minimal, since at that point almost all
necessary resources are cached by and served from the ServiceWorker.

Fixes #17539

aio/src/testing/sw-update-notifications.service.ts

@@ -1,4 +0,0 @@
-export class MockSwUpdateNotificationsService {
-  enable = jasmine.createSpy('MockSwUpdateNotificationsService.enable');
-  disable = jasmine.createSpy('MockSwUpdateNotificationsService.disable');
-}

aio/src/testing/sw-updates.service.ts

@@ -1,20 +0,0 @@
-import { Subject } from 'rxjs/Subject';
-import 'rxjs/add/operator/distinctUntilChanged';
-import 'rxjs/add/operator/take';
-
-
-export class MockSwUpdatesService {
-  $$activateUpdateSubj = new Subject<boolean>();
-  $$isUpdateAvailableSubj = new Subject<boolean>();
-  isUpdateAvailable = this.$$isUpdateAvailableSubj.distinctUntilChanged();
-
-  activateUpdate(): Promise<boolean> {
-    return new Promise(resolve => {
-      this.$$activateUpdateSubj
-          // Better simulate what actually happens with the real ServiceWorker.
-          .take(1)
-          .do(() => this.$$isUpdateAvailableSubj.next(false))
-          .subscribe(resolve);
-    });
-  }
-}