prevent overlays on the phone settings

A report came in showing a 3rd party app could overlay a button
on the phone settings causing unwanted behavior.  In order to prevent
this, a new system flag has been added that only allows system overlays.

bug: 246933785
Test: manual
Change-Id: I427b65bc6c1acf06676e1753a34a7a38e21bbae0
Merged-In: I427b65bc6c1acf06676e1753a34a7a38e21bbae0
(cherry picked from commit e827d8f13c1c92622474fa2bf9e41a1f4ce21e2c)
Merged-In: I427b65bc6c1acf06676e1753a34a7a38e21bbae0