child user: Validate parent access code entered on lock screen

Create an instance of ParentAccessService in ScreenTimeController.
Unblock lock screen upon successful validation of access code.

Bug: 911326
Change-Id: If304be20ad59fe1e9a108faa1f1c84e179a97c91
Reviewed-on: https://chromium-review.googlesource.com/c/1474544
Commit-Queue: Aga Wronska <agawronska@chromium.org>
Reviewed-by: Jacob Dufault <jdufault@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635706}

• ash/login/login_screen_controller.cc[diff]
• ash/login/login_screen_controller.h[diff]
• ash/login/ui/lock_contents_view.cc[diff]
• ash/login/ui/lock_contents_view.h[diff]
• ash/login/ui/login_big_user_view.cc[diff]
• ash/login/ui/parent_access_view.cc[diff]
• ash/login/ui/parent_access_view.h[diff]
• ash/login/ui/parent_access_view_unittest.cc[diff]
• chrome/browser/chromeos/child_accounts/parent_access_code/parent_access_service.cc[diff]
• chrome/browser/chromeos/child_accounts/parent_access_code/parent_access_service.h[diff]
• chrome/browser/chromeos/child_accounts/screen_time_controller.cc[diff]
• chrome/browser/chromeos/child_accounts/screen_time_controller.h[diff]
• chrome/browser/prefs/browser_prefs.cc[diff]