[privacy view] Fix a11y traversal for the privacy notice.

There is currently a bug where the privacy notice can be navigated with
tab and arrow keys, but not by the ChromeVox controls such as
search+right. This is due to ChromeVox being incompatible with nested
buttons.

This CL does two things:
1) Reassign the outer view to a non-button a11y role, allowing ChromeVox
   to access the inner views as well.
2) Disable notifications for the outer view to avoid duplicated
   notifications.

Bug: 11127142
Change-Id: Ibe7dca2c9f183f824ec1f410c6511322a307564e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2397111


Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Commit-Queue: Rachel Wong <wrong@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805191}