[App Management] Add borders between permission toggles.

This CL adds boarders between permission toggles to help guide the user
when App Management is being used on a device with a touch screen.

Before (without border): https://bugs.chromium.org/p/chromium/issues/attachment?aid=413406&signed_aid=gaumzEtlv1JcHLrQECRcqQ==&inline=1
After (with border): https://bugs.chromium.org/p/chromium/issues/attachment?aid=413407&signed_aid=L9yUkITEkaSUjJ4iHODEjg==&inline=1

Bug: 1007163
Change-Id: I6410e51bbf119879a5376af698b5fba15c86c050
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1820499
Auto-Submit: Jeevan Shikaram <jshikaram@chromium.org>
Reviewed-by: Alan Cutter <alancutter@chromium.org>
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699796}

chrome/browser/resources/settings/chromeos/os_apps_page/app_management_page/shared_style.html

@@ -80,9 +80,14 @@
       }
 
       .subpermission-row {
+        border-top: var(--card-separator);
         height: 48px;
       }
 
+      .subpermission-row:first-of-type {
+        border-top: none;
+      }
+
       .secondary-text {
         color: var(--secondary-text-color);
         font-weight: var(--secondary-font-weight);