fix(aria/accordion): ensure right attribute is set when used as a host directive (#33839)

Some functionality in the `AccordionGroupPattern` relies on the `ngAccordionTrigger` attribute being present. These changes add it as an explicit host binding in case the directive is used as a host directive.

Author: crisbeto

src/aria/accordion/accordion-trigger.ts

@@ -52,6 +52,11 @@ import {AccordionPanel} from './accordion-panel';
     '[attr.aria-disabled]': '_pattern.disabled()',
     '[attr.disabled]': '_pattern.hardDisabled() ? true : null',
     '[attr.tabindex]': '_pattern.tabIndex()',
+
+    // `AccordionGroupPattern` depends on this to find the closest
+    // trigger so ensure that it is set. It's usually there for normal usage,
+    // but it might not be when the trigger is a host directive.
+    'ngAccordionTrigger': '',
   },
 })
 export class AccordionTrigger implements OnInit, OnDestroy {