diagnostics: Launch app directly on a page

ARC optin dialog wants to launch the Diagnostics App in case of
connectivity failures so it makes sense to directly navigate to
connectivity page of the diagnostics app instead of the overview.
In this change, we're adding an argument to the
DiagnosticsDialog::ShowDialog api to indicate the page we want to launch
and it will send a query parametre to the app for that page.

Bug: 1228406
Test: deploy to DUT, cause optin failure, and launch the app
Change-Id: I8cdb53a2e88c1be4efdd686685a3718c107ed63a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3134420
Commit-Queue: Muhammad Hasan Khan <mhasank@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Josh Horwich <jhorwich@chromium.org>
Cr-Commit-Position: refs/heads/main@{#917269}