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
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:df163f70fd3f456604019072b796eaeab71418ae)
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:595c611192c340588278b88686e095b13e883929)
Merged-In: I6c0080e8205410f4b6a389b793d56b63ebcc5e95
Change-Id: I6c0080e8205410f4b6a389b793d56b63ebcc5e95
1 file changed