GitLab

Adds DetachableBaseNotificationController that observes
DetachableBaseHandler in order to detect when a detachable base
different than the last base used by the active user changes. When a
detachable base change is detected, it shows a system notification.
The notification serves a security purpose - it warns the user that
the detachable base they are using has changed, and that the base could
be compromised (to steal their keystrokes).

Note that the notification is shown only if the session is unblocked
at the time - lock screen and login screen will implement their own UI
to notify the user about the base change.

If/when the session is unblocked, the attached base is set as the last
one used by the user to prevent the notification from showing up next
time the user attaches the same base.

Bug: 796300
Change-Id: I7ff8fdf3e02e5833df2a709169fdd076285f3a4b
Reviewed-on: https://chromium-review.googlesource.com/952477Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Commit-Queue: Toni Barzic <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541717}