Add hooks for displaying Account Manager welcome screen

It will be displayed when:
  - Users click on the Secondary Account reauth notification.
  - Users visit the Account Manager UI.

Additonally, the welcome screen will not be shown if it has been shown
once before already.

Bug: 919251
Test: browser_tests --gtest_filter="*CrSettingsPeoplePageAccountManagerTest*"
Change-Id: I41f5a9aaf64d822401a2ed02d9a78ca3b6a77cf5
Reviewed-on: https://chromium-review.googlesource.com/c/1422001
Commit-Queue: Kush Sinha <sinhak@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624464}
10 files changed