[Switch Access] Add support for nested buttons

This solves an issue where a button element that contains other controls
cannot be accessed using Switch Access, as the previous paradigm forced
each node to be either a group or actionable.

The new icons can be seen here:
https://screenshot.googleplex.com/7ikBinJt8gGpDaD
https://screenshot.googleplex.com/3zxgjHyUPe6vQku

AX-Relnotes: Supports nested buttons with Switch Access.
Fixed: b/345447869
Test: SwitchAccessBasicNodeTest.Actions
Change-Id: Ica19f8d77b50f3e0a1125cf1c06291fc7bea6014
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5698349
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Anastasia Helfinstein <anastasi@google.com>
Reviewed-by: Akihiro Ota <akihiroota@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Auto-Submit: Anastasia Helfinstein <anastasi@google.com>
Cr-Commit-Position: refs/heads/main@{#1328464}