fix(material/tree): add aria-expanded to ngOnInit

added aria-expanded to ngOnInit to get rid of ExpressionChangedAfterItHasBeenCheckedError

Fixes #21922

Author: DBowen33

src/cdk/tree/tree.ts

@@ -321,7 +321,7 @@ export class CdkTree<T, K = T> implements AfterContentChecked, CollectionViewer,
   exportAs: 'cdkTreeNode',
   host: {
     'class': 'cdk-tree-node',
-    '[attr.aria-expanded]': 'isExpanded',
+    '[attr.aria-expanded]': 'isLeafNode ? null : isExpanded',
   },
   standalone: true,
 })
@@ -388,10 +388,7 @@ export class CdkTreeNode<T, K = T> implements FocusableOption, OnDestroy, OnInit
     return false;
   }
 
-  get isExpanded(): boolean | null {
-    if (this.isLeafNode) {
-      return null;
-    }
+  get isExpanded(): boolean {
     return this._tree.treeControl.isExpanded(this._data);
   }
 
@@ -415,6 +412,11 @@ export class CdkTreeNode<T, K = T> implements FocusableOption, OnDestroy, OnInit
   ngOnInit(): void {
     this._parentNodeAriaLevel = getParentNodeAriaLevel(this._elementRef.nativeElement);
     this._elementRef.nativeElement.setAttribute('aria-level', `${this.level + 1}`);
+    if (!this.isLeafNode) {
+      this._elementRef.nativeElement.setAttribute('aria-expanded', this.isExpanded.toString());
+    } else {
+      this._elementRef.nativeElement.removeAttribute('aria-expanded');
+    }
   }
 
   ngOnDestroy() {