[Bluetooth] a11y: Alert user when dialog status changes.

Alert the user when the Bluetooth device is being connected to, requires
passkey entry or confirmation, or fails to connect (an existing
notification already alerts the user when the device successfully
connects).

Bug: 1007364
Change-Id: Icbca5b4ff3c5c645c4e21f17ef040acb107e9352
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1876470Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Ryan Hansberry <hansberry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709287}

ui/webui/resources/cr_components/chromeos/bluetooth_dialog.html

@@ -83,7 +83,8 @@
         on-closed="onDialogCanceled_" exportparts="dialog">
       <div slot="title">[[dialogTitle]]</div>
       <div slot="body">
-        <div class="contents layout vertical center center-justified">
+        <div class="contents layout vertical center center-justified"
+            role="alert">
           <template is="dom-if" if="[[!errorMessage_]]">
             <div id="pairing" class="layout vertical center center-justified">
               <div class="dialog-message">