Bug: 21589105 Rescope WRITE_SETTINGS and SYSTEM_ALERT_WINDOW permission

Raised the protection level of WRITE_SETTINGS permission to appop and also
allowed backwards compatibility with pre23 flag. Also made sure that this
permission is not added as RuntimePermission in DefaultPermissionGrantPolicy as
that breaks a lot of gmscore stuff.

Introduced new action to manage write system settings as a new API and
renamed the string that describes the managing of overlay permission.

Change-Id: Ifd25a6ddc06de68ee37015cb9fb23452e4ef10f4