Skip TestDropPacketToVpnAddress if the feature is disabled

INGRESS_TO_VPN_ADDRESS_FILTERING is disabled due to b/352424251 and
testDropPacketToVpnAddress fails if the feature is disabled.
So skip testDropPacketToVpnAddress if the feature is disabled.

Bug: 362434644
Test: adb shell device_config put tethering
      ingress_to_vpn_address_filtering -1
Test: atest CtsHostsideNetworkTests

Change-Id: Ib16a7550dcf93dd9ecf29047b1c4f101ff6d610d

service/src/com/android/server/ConnectivityService.java

@@ -14505,4 +14505,14 @@ public class ConnectivityService extends IConnectivityManager.Stub
         }
         return features;
     }
+
+    @Override
+    public boolean isConnectivityServiceFeatureEnabledForTesting(String featureFlag) {
+        switch (featureFlag) {
+            case INGRESS_TO_VPN_ADDRESS_FILTERING:
+                return mIngressToVpnAddressFiltering;
+            default:
+                throw new IllegalArgumentException("Unknown flag: " + featureFlag);
+        }
+    }
 }