[CrOS Settings] Focus button when returning from a11y subpages

This CL fixes an a11y issue where, on the ChromeOS Settings app >
"Accessibility" > "Manage accessibility features" page,
focus on buttons would be lost when navigating back from a subpage
(e.g. "Captions") using the keyboard.

Ultimately, the issue was that the underlying component
for this page, `settings-manage-a11y-page`, overrides
`currentRouteChanged()` which replaces the default logic implemented in
`RouteOriginBehavior` to focus a selector when the page is changed.

https://chromium-review.googlesource.com/c/chromium/src/+/3362720
was a good reference CL.

Bug: 1227067
Change-Id: I832263cc2300134acefa10db72cd0814463a59a3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3486163
Reviewed-by: David Tseng <dtseng@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: Wes Okuhara <wesokuhara@google.com>
Cr-Commit-Position: refs/heads/main@{#978525}