[Nearby] Add subtitle to device visibility dialog.

This updates the device visibility dialog to add a subtitle and adjust
the font weight of the title to match the specs.

Screenshot:
  https://screenshot.googleplex.com/3MnGaYgLuaT4L7H.png

Strings spec:
  https://docs.google.com/document/d/1s-KjC1mM4ZPYeYAO7oI8lXltS_1FaYYfMMnNDdbfRag

Bug: b:168841167
Change-Id: Ied28371e97bc196ba051c95d0a8a4ee12b7400c4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2515144Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Michael Hansen <hansenmichael@google.com>
Cr-Commit-Position: refs/heads/master@{#823438}

chrome/browser/resources/settings/chromeos/nearby_share_page/nearby_share_contact_visibility_dialog.html

@@ -15,8 +15,24 @@
 
 <dom-module id="nearby-share-contact-visibility-dialog">
   <template>
+    <style>
+      div.title {
+        font-weight: 500;
+        padding-bottom: 0;
+      }
+
+      div.header {
+        padding-bottom: 20px;
+        padding-inline-start: 20px;
+      }
+    </style>
     <cr-dialog id="dialog" show-on-attach>
-      <div slot="title">$i18n{nearbyShareVisibilityDialogTitle}</div>
+      <div slot="title" class="title">
+        $i18n{nearbyShareVisibilityDialogTitle}
+      </div>
+      <div slot="header" class="header">
+        $i18n{nearbyShareVisibilityPageSubtitle}
+      </div>
       <div slot="body">
         <nearby-contact-visibility settings="{{settings}}">
         </nearby-contact-visibility>