do not drop ingress dns replies with non bypassable vpn

See:
  aosp-master packages/modules/DnsResolver/include/netd_resolv/resolv.h
  119: #define TAG_SYSTEM_DNS 0xFFFFFF82

  aosp-master frameworks/base/core/java/com/android/internal/util/TrafficStatsConstants.java
  43: public static final int TAG_SYSTEM_DNS = 0xFFFFFF82;

  aosp-master packages/modules/DnsResolver/resolv_private.h
  171: if (int err = android::net::gResNetdCallbacks.tagSocket(sock, TAG_SYSTEM_DNS, uid, pid)) {

Test: via new tests on aosp master & via test build with carrier
Bug: 159994981
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I8765153c1d7b10970f48bd7e85780f24d2549190