vpn: allow IPSec traffic through Always-on VPN

This won't leak any traffic outside the VPN as long as there are no
processes owned by uid 0 which generate network traffic (which is
currently the case).

Bug: 69873852
Test: compared the output of 'adb shell ip rule show' before and after
Test: runtest -x frameworks/base/tests/net/java/com/android/server/connectivity/VpnTest.java
Test: local CTS tests run: android.net.cts.VpnServiceTest
Test: local CTS tests run: com.android.cts.devicepolicy.MixedDeviceOwnerTest
Change-Id: I8758e576c9d961d73f62bfcf0559dd7ecee6e8e6