MOBILE-4270 collapsible-header: Fix changes not detected

Before this change, listenEvents was called after initializeFloatingTitle, so the mutationObserver didn't exist when initializeFloatingTitle was executed.

src/core/directives/collapsible-header.ts

@@ -125,6 +125,8 @@ export class CoreCollapsibleHeaderDirective implements OnInit, OnChanges, OnDest
             await this.enteredPromise,
         ]);
 
+        this.listenEvents();
+
         await this.initializeFloatingTitle();
         this.initializeContent();
     }
@@ -291,8 +293,6 @@ export class CoreCollapsibleHeaderDirective implements OnInit, OnChanges, OnDest
             return;
         }
 
-        this.listenEvents();
-
         // Initialize from tabs.
         const tabs = CoreDirectivesRegistry.resolve(this.page.querySelector('core-tabs-outlet'), CoreTabsOutletComponent);