Skip adding ingress discard rule to legacy VPN

Cherry-pick of aosp/3201971 to backport VPN security fix to non-mainline
U devices.

Some legacy VPNs need to receive packets to VPN address via
non-VPN interface.

Bug: 193031925
Test: TH
Merged-In: Iad06dc69c5bf5ccda476e89ed9b0fc1bc63d2ebe
Change-Id: If4f6b095a719a0abcb6254c522beac5d45110d4d
1 file changed