NetworkPolicyManager: Add POLICY_REJECT_ALL constant

* This was used by network-isolation in 11 and earlier versions
* We've switched to restricted-networking-mode for 12, however
  we should still keep this around for usage with migration, and
  as a reminder that this value should not be re-used to avoid
  future complications
* This is also used for backup and restore now

Change-Id: I1e51cc610ecc8cd0cb8f86b25f9f990f348b4097