fix(accordion): prevent error with nested accordions (#1120)

This is a backport of 898e1df (#1119) to 15.x.

An instance of the accordion component should only care about its own
panels. Including nested accordions' panels causes an error when trying
to update panels that don't belong the parent instance.

closes #1118

projects/angular/src/accordion/accordion.ts

@@ -32,7 +32,7 @@ import { AccordionService } from './providers/accordion.service';
 })
 export class ClrAccordion implements OnInit, OnChanges, AfterViewInit, OnDestroy {
   @Input('clrAccordionMultiPanel') multiPanel: boolean | string = false;
-  @ContentChildren(ClrAccordionPanel, { descendants: true }) panels: QueryList<ClrAccordionPanel>;
+  @ContentChildren(ClrAccordionPanel) panels: QueryList<ClrAccordionPanel>;
   subscriptions: Subscription[] = [];
 
   constructor(private accordionService: AccordionService) {}