fix(slide-toggle): clear tabindex from host element (#13311)

Along the same lines as #13308. Currently we forward the `tabindex` from the host to the underlying `input`, however we never clear the `tabindex` from the host, which means that we end up with two layers of focusable elements.

src/lib/slide-toggle/slide-toggle.spec.ts

@@ -348,6 +348,15 @@ describe('MatSlideToggle without forms', () => {
       expect(slideToggle.tabIndex)
         .toBe(5, 'Expected tabIndex property to have been set based on the native attribute');
     }));
+
+    it('should clear the tabindex from the host element', fakeAsync(() => {
+      const fixture = TestBed.createComponent(SlideToggleWithTabindexAttr);
+
+      fixture.detectChanges();
+
+      const slideToggle = fixture.debugElement.query(By.directive(MatSlideToggle)).nativeElement;
+      expect(slideToggle.getAttribute('tabindex')).toBeFalsy();
+    }));
   });
 
   describe('custom action configuration', () => {

src/lib/slide-toggle/slide-toggle.ts

@@ -85,6 +85,7 @@ export const _MatSlideToggleMixinBase:
   host: {
     'class': 'mat-slide-toggle',
     '[id]': 'id',
+    '[attr.tabindex]': 'null',
     '[class.mat-checked]': 'checked',
     '[class.mat-disabled]': 'disabled',
     '[class.mat-slide-toggle-label-before]': 'labelPosition == "before"',