System settings can be changed by system apps.

The system settings permission is going away and the user will
be able to choose which apps have access to system settings in
the UI. So, if an app is white-listed by the user or being on
the system image, we allow access to system settings. Also if
the caller has the stronger write secure settings permission
we allow changes of the less sensitive system settings.

Change-Id: I7aca958fd0ad2c588117b8c6e44d78eb16d648bc