fix(aio): strip leading slashes from path (and improve DRY-ness) (#16238)

Previously, the path returned by `LocationService.path()` preserved leading
slashes, which resulted in requests with consequtive slashes in the URL. Such
requests would fail (with a 404) on staging.

This commit fixes it, by removing leading slashes from the path. It also
refactors `LocationService` a bit, converting path to an observable,
`currentPath` (similar to `currentUrl`), and applies certain clean-ups (e.g.
stripping slashes, query, hash) in one place, which simplifies consumption.

Closes #16230

aio/src/app/embedded/current-location.component.ts

@@ -7,7 +7,7 @@ import { LocationService } from 'app/shared/location.service';
  */
 @Component({
   selector: 'current-location',
-  template: '{{location.path()}}'
+  template: '{{ location.currentPath | async }}'
 })
 export class CurrentLocationComponent {
   constructor(public location: LocationService) {