Add implementation of getInterfaceHash()

Exempt-From-Owner-Approval: cherry-pick from internal
Bug: 136065010
Test: m
Change-Id: I314f19aeca82cc8653eab71c9526ea7a208e6b50
Merged-In: I314f19aeca82cc8653eab71c9526ea7a208e6b50

Tethering/src/android/net/ip/IpServer.java

@@ -301,6 +301,11 @@ public class IpServer extends StateMachine {
         public int getInterfaceVersion() {
             return this.VERSION;
         }
+
+        @Override
+        public String getInterfaceHash() {
+            return this.HASH;
+        }
     }
 
     private class DhcpServerCallbacksImpl extends DhcpServerCallbacks {