Fix crash when session is restarted for child user

During session restart ScreenTimeController was creating
ParentAccessService that depends on the LoginScreenClient before
LoginScreenClient was created. Lazy initialize ParentAccessService to
prevent crash.

Bug: 956375
Test: CrashRestoreChildUserTest
Change-Id: I118e80b06297ae098f22e5132bc6d03b0ae21b4b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1585073
Commit-Queue: Aga Wronska <agawronska@chromium.org>
Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654317}
12 files changed