[Tether03] Migrate IpServer into module

Add IpServer which is used to serve ip configuration, dhcp, dns proxy
and nat for downstream interface.

Bug: 136040414
Test: -build, flash, boot
      -atest TetheringTests
      -atest FrameworksNetTests

Change-Id: I23652ae0b9509abe7d38da96d523eb22ab00a343

Tethering/src/android/net/util/InterfaceSet.java

@@ -0,0 +1,52 @@
+/*
+ * Copyright (C) 2018 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package android.net.util;
+
+import java.util.Collections;
+import java.util.HashSet;
+import java.util.Set;
+import java.util.StringJoiner;
+
+
+/**
+ * @hide
+ */
+public class InterfaceSet {
+    public final Set<String> ifnames;
+
+    public InterfaceSet(String... names) {
+        final Set<String> nameSet = new HashSet<>();
+        for (String name : names) {
+            if (name != null) nameSet.add(name);
+        }
+        ifnames = Collections.unmodifiableSet(nameSet);
+    }
+
+    @Override
+    public String toString() {
+        final StringJoiner sj = new StringJoiner(",", "[", "]");
+        for (String ifname : ifnames) sj.add(ifname);
+        return sj.toString();
+    }
+
+    @Override
+    public boolean equals(Object obj) {
+        return obj != null
+                && obj instanceof InterfaceSet
+                && ifnames.equals(((InterfaceSet) obj).ifnames);
+    }
+}