Skip adding ingress discard rule to OEM VPN

Cherry-pick of aosp/3208090 to backport VPN security fix to non-mainline
U devices.

OEM VPNs might need to receive packets to VPN address via
non-VPN interface.

Bug: 193031925
Test: TH
Merged-In: Ia9000bcbb5b3b9e1e0d396ba78946be36e2fc5ba
Change-Id: I6c0080e8205410f4b6a389b793d56b63ebcc5e95
1 file changed