editor: add observer for native panel

This CL adds an observer of the ash backend for the editor native panel.
This observer will be used in future CLs to listen for relevant state
changes and update the native panel accordingly.

BUG=b:324096105

Change-Id: I6d8211f2dc0fbc396b833d9f0827e3da4cb5b923
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5315658
Reviewed-by: Darren Shen <shend@chromium.org>
Reviewed-by: Sam McNally <sammc@chromium.org>
Commit-Queue: Curtis McMullan <curtismcmullan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1268762}