Remove the old selected view controller from the view hierarchy when …

…switching selected view controller, and add accessibility post notifications when switching selected view controllers so that VoiceOver will always switch to a new element visible on screen.

PiperOrigin-RevId: 327825660

components/Tabs/src/MDCTabBarViewController.m

@@ -210,6 +210,10 @@ - (void)updateOldSelectedViewController:(nullable UIViewController *)oldSelected
     NSAssert([_viewControllers containsObject:selectedViewController], @"not one of us.");
   }
 
+  [oldSelectedViewController willMoveToParentViewController:nil];
+  [oldSelectedViewController.view removeFromSuperview];
+  [oldSelectedViewController removeFromParentViewController];
+
   if (![self.childViewControllers containsObject:selectedViewController]) {
     [self addChildViewController:selectedViewController];
     UIView *view = selectedViewController.view;
@@ -231,6 +235,8 @@ - (void)updateOldSelectedViewController:(nullable UIViewController *)oldSelected
     self.tabBar.selectedItem = selectedViewController.tabBarItem;
   }
   [self setNeedsStatusBarAppearanceUpdate];
+
+  UIAccessibilityPostNotification(UIAccessibilityScreenChangedNotification, nil);
 }
 
 - (void)setSelectedViewController:(nullable UIViewController *)selectedViewController {