Support disabled/log/enforce in ro.control_privapp_permissions

Possible values:
 - disable - the feature is completely disabled - signature|privileged
   permissions are granted automatically without logging. Default behavior
 - log - log grants of signature|privileged permissions to privileged apps
   that were not whitelisted in /etc/permissions/privapp-permissions.xml.
 - enforce - enforce that only whitelisted permissions are granted.
   Only devices with ro.control_privapp_permission=enforce will pass CTS tests.

Test: Manual - device boots, permissions are granted
Bug:31008485
Change-Id: I24e0c75d2efc326c9fbec35efc92489313667212