Add DDR feature flag

Bug: 240259333
Test: build pass
Change-Id: I520f3923ab3c47a7b7f62b9c242d42c113e71f5d
diff --git a/src/com/android/networkstack/util/NetworkStackUtils.java b/src/com/android/networkstack/util/NetworkStackUtils.java
index 26a83bb..2906702 100755
--- a/src/com/android/networkstack/util/NetworkStackUtils.java
+++ b/src/com/android/networkstack/util/NetworkStackUtils.java
@@ -267,6 +267,11 @@
     public static final String IPCLIENT_DHCPV6_PD_PREFERRED_FLAG_VERSION =
             "ipclient_dhcpv6_pd_preferred_flag_version";
 
+    /**
+     * Experiment flag to enable Discovery of Designated Resolvers (DDR).
+     */
+    public static final String DNS_DDR_VERSION = "dns_ddr_version";
+
     /**** BEGIN Feature Kill Switch Flags ****/
 
     /**