Add support for secure system overlays.

Manual merge from Gingerbread.

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: Id876736fd8bf332ff9a5428bde59f5268aa49c3a