[iOS] Add accessibility trait to disabled search engines

This CL adds the UIAccessibilityTraitNotEnabled to the search engine
cells when they are not interactable.

Fixed: 1040497
Change-Id: Iec65f625d7bf3d7f738d82ab86b204c3c58a6099
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1994284
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Robbie Gibson <rkgibson@google.com>
Auto-Submit: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Robbie Gibson <rkgibson@google.com>
Cr-Commit-Position: refs/heads/master@{#730133}

ios/chrome/browser/ui/settings/cells/search_engine_item.mm

@@ -54,9 +54,11 @@
   if (self.enabled) {
     cell.contentView.alpha = 1.0;
     cell.userInteractionEnabled = YES;
+    cell.accessibilityTraits &= ~UIAccessibilityTraitNotEnabled;
   } else {
     cell.contentView.alpha = 0.4;
     cell.userInteractionEnabled = NO;
+    cell.accessibilityTraits |= UIAccessibilityTraitNotEnabled;
   }
 
   if (styler.cellTitleColor) {