[CrOS Settings] Add visual focus indicator to picture selection UI

Add a visual outline around the currently focused image.

Screenshot:
https://screenshot.googleplex.com/9Bwy5ZFcFoVuWX4.png

Bug: 702730
Change-Id: I9c091d022ed3f230b99894fc35c1afb3c807c075
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2476627
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Gordon Seto <gordonseto@google.com>
Cr-Commit-Position: refs/heads/master@{#818152}

ui/webui/resources/cr_elements/chromeos/cr_picture/cr_picture_list.html

@@ -31,6 +31,10 @@
         outline: none;
       }
 
+      img:focus {
+        outline: 1px solid rgba(66, 133, 244, 0.6);
+      }
+
       iron-icon {
         --iron-icon-fill-color: var(--google-grey-500);
         --iron-icon-height: 24px;
@@ -50,8 +54,8 @@
         outline: none;
       }
 
-      #container {
-        outline: none;
+      iron-icon:focus {
+        outline: 1px solid rgba(66, 133, 244, 0.6);
       }
     </style>