NetworkPolicyManagerService: Fix network isolation for secondary users

* 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: Id598264c965aafade8e79b9eeca608711ac49028