[views-ax] Implement ViewAccessibility serialization

This CL enables basic serialization of ViewAccessibility objects by
fixing critical issues in WidgetViewAXCache, the backbone of the
serialization path.

Previously, the cache did not:
1. Provide immutable child snapshots for AXTreeSource, instead reading live children directly, risking inconsistencies during serialization.
2. Accurately track nodes in the view hierarchy, failing to remove stale entries and inconsistently adding new ones, causing the map to grow unchecked.

This CL addresses both by:
* Introducing proper child snapshots used during serialization.
* Updating the node map on view add/remove.
* Seeding the cache with the full subtree on WidgetAXManager::Enable().

A follow-up CL will ensure the root view of the root widget is always
serialized, not only after accessibility is enabled.

Bug: 40672441
Change-Id: I617e2b73317c48c9d9e59534c6fee6b21405af78
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6986683
Commit-Queue: Benjamin Beaudry <benjamin.beaudry@microsoft.com>
Reviewed-by: Jacques Newman <janewman@microsoft.com>
Auto-Submit: Benjamin Beaudry <benjamin.beaudry@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1524969}