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.

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: I939a95b9a8b1ae75bf67aa6d8190c92a8d2001d3