[AriaNotify] Rename AriaNotificationMessage for consistency

1. The `ariaNotify` API specifies `announcement` as the name of its
   first input string. This was inconsistent with its respective string
   attribute in `AXNodeData` which was named `AriaNotificationMessage`.

   Renaming it `AriaNotificationAnnouncement` to match the API.

2. Introducing getters and setters for `AriaNotification` attributes.

Bug: 326277796
Change-Id: If7d7fbd2338abcd05137bdad6ca79312f7d0f75d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5324894
Commit-Queue: Ethan Jimenez <ethavar@microsoft.com>
Reviewed-by: Benjamin Beaudry <benjamin.beaudry@microsoft.com>
Reviewed-by: Sam McNally <sammc@chromium.org>
Reviewed-by: Alison Maher <almaher@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1266156}