VoiceOver pre-caches live region data and does its own deltas to
know what to parts of a subtree changed, and what to announce
based on AXARIAAtomic and AXARIARelevant.
I added a removed event as well. This will help us cache a "live region"
flag in the main process and avoid sync round trips for attributes when not needed.
Differential Revision: https://phabricator.services.mozilla.com/D96291