Add a notifier class for usage time state.

This notifier should call the observers when the child is using the
device, allowing us to know when the usage time should be counting.
This will be useful in the future because we will need to listen to the
same observers in ScreenTimeController.

Bug: 919132
Change-Id: I27577eac0cf59de615d5343689d5d9fe2e1f781c
Reviewed-on: https://chromium-review.googlesource.com/c/1443831
Commit-Queue: Leonardo Padilha <ldaguilar@chromium.org>
Reviewed-by: Aga Wronska <agawronska@chromium.org>
Reviewed-by: Sergey Poromov <poromov@chromium.org>
Reviewed-by: Henrique Grandinetti <hgrandinetti@chromium.org>
Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629335}
9 files changed