[ViewsAX] Move ScopedObservation to AXTreeManager

In this CL, we make the AXTreeManager registers itself as an
AXTreeObserver on its AXTree. We use base::ScopedObservation so that
it removes itself from the tree's observers list automatically at
destruction.

Bug: 1049261
Change-Id: Ic30eff5373fa50689a19db9372f7e91b0addc2a8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3817182
Commit-Queue: Benjamin Beaudry <benjamin.beaudry@microsoft.com>
Reviewed-by: Daniel Libby <dlibby@microsoft.com>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1034682}