The OAuthService doesn't clean up all used resources when it is destroyed

[Andreas-Hjortland]

Describe the bug
If you destroy the OAuthService, for instance through destroying the angular app, the attached event listeners and the sessionCheckTimer will still continue to run.

Stackblitz example
https://angular-abivfc.stackblitz.io/

To Reproduce
Steps to reproduce the behavior:

1. Go to the stackblitz. You might need to change of fix the issuer configured in app.component.ts. I have used the one in the example code.
2. Log in to the identity server and redirect back to the site
3. Open the console
4. See error "Cannot read proprety 'contentWindow' of null" or similar error on other browsers.

Expected behavior
All timers, events and elements that the OAuthService created should be stopped or removed when the angular app is destroyed and the framework calls ngOnDestroy.

Desktop (please complete the following information):

• OS: Tested on windows 10, but will probably work the same on all platforms
• Browser Tested in chrome 78 and Firefox 71

[Andreas-Hjortland]

The issue isn't that you get the "Cannot read property 'contentWindow' of null", but rather a symptom of the underlying issue that we haven't cleaned up all the resources that we allocate