Allow smart unlock right after boot.

This change enables relaxing the constraint by which
strong authentication is needed after device reboot.

There are very limited use cases where this might be
safe, one of which is in a car. Cars head units usually
are protected physically by the car and have hardware
anti-theft mechanisms so we can potentially allow for
Android to allow users to use smart unlock to avoid the
lockscreen just after boot.

This change adds in a config flag that sets the default
trust flags, which can be set to allow smart unlock after
boot for car head units.

Bug: 26559008
Change-Id: Id6338a97b617ddaf3d2fae5d51235429a42b81cc