fix(material/list): indexOf usage incorreect for active focus reset (#28531)

The buggy logic was using a truthiness check for checking for presence
in a list, but this check needed to compare against -1 instead.  This
corrects a mismanagement of tabIndex on the items in the selection
list: if the list of items changed, the selection list could not
be tab selected until the selection list was mouse-focused.

Author: dyoo

src/material/list/selection-list.spec.ts

@@ -1319,6 +1319,21 @@ describe('MDC-based MatSelectionList with forms', () => {
         .toBe(1);
     }));
 
+    it('should focus the first option when the list items are changed', fakeAsync(() => {
+      fixture.componentInstance.options = ['first option', 'second option'];
+      fixture.detectChanges();
+
+      tick();
+
+      const listElements = fixture.debugElement
+        .queryAll(By.directive(MatListOption))
+        .map(optionDebugEl => optionDebugEl.nativeElement);
+
+      expect(listElements.length).toBe(2);
+      expect(listElements[0].tabIndex).toBe(0);
+      expect(listElements[1].tabIndex).toBe(-1);
+    }));
+
     it('should not mark the model as touched when the list is blurred', fakeAsync(() => {
       expect(ngModel.touched)
         .withContext('Expected the selection-list to be untouched by default.')

src/material/list/selection-list.ts

@@ -429,7 +429,7 @@ export class MatSelectionList
     this._items.changes.pipe(takeUntil(this._destroyed)).subscribe(() => {
       const activeItem = this._keyManager.activeItem;
 
-      if (!activeItem || !this._items.toArray().indexOf(activeItem)) {
+      if (!activeItem || this._items.toArray().indexOf(activeItem) === -1) {
         this._resetActiveOption();
       }
     });