New permission to manage device admins

The previously used permission was doing double duty as the permission
that device admins to check for to ensure that calls are coming from valid
system components.

MANAGE_DEVICE_ADMINS is system|signature and is now required to add/remove
device admins.

Required for:
Bug: 9856348

Change-Id: I64385d2ec734c3957af21b5a5d9cffd8a3bcd299