Clear calling identity when setting restricted networking mode UID firewall rules

* NetworkManager setFirewallUidRule checks that the caller is system uid

* Public service entry points are already protected with MANAGE_NETWORK_POLICY permission so simply clear calling identity around NetworkPolicyManagerService setUidFirewallRule() call to resolve crash for secondary users during settings change.

Change-Id: I2fb22e77c0afa67acfbb5b9d57173df5aefb0d57