Add support for secure system overlays. (DO NOT MERGE)

This change adds a new window type for secure system overlays
created by the system itself from non-secure system overlays that
might be created by applications that have the system alert permission.
Secure views ignore the presence of secure system overlays.

Bug: 3098519
Change-Id: I8f8398f4fdeb0469e5d71124c21bedf121bd8c07