fix(platform-browser/testing): clean up public api for platform-browser/testing (#9519)

Mostly, removing things that were never intended to be exported publicy.

BREAKING CHANGE:

The following are no longer publicly exported APIs. They were intended as internal
utilities and you should use your own util:

```
browserDetection,
dispatchEvent,
el,
normalizeCSS,
stringifyElement,
expect (and custom matchers for Jasmine)
```

modules/@angular/platform-browser/test/security/html_sanitizer_spec.ts

@@ -7,7 +7,7 @@
  */
 
 import * as t from '@angular/core/testing/testing_internal';
-import {browserDetection} from '@angular/platform-browser/testing';
+import {browserDetection} from '@angular/platform-browser/testing/browser_util';
 
 import {getDOM} from '../../src/dom/dom_adapter';
 import {sanitizeHtml} from '../../src/security/html_sanitizer';