Fix some @hide usage in Tethering

Using alternative way to replace some @hide API.

Bug: 144814072
Test: build, flash, boot
      atest TetheringTests

Change-Id: I1e12d69db1ad91dff553e142e17c6a70808e1639

packages/Tethering/src/android/net/util/TetheringMessageBase.java

@@ -0,0 +1,25 @@
+/*
+ * Copyright (C) 2020 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;
+
+/**
+ * This class defines Message.what base addresses for various state machine.
+ */
+public class TetheringMessageBase {
+    public static final int BASE_MASTER   = 0;
+    public static final int BASE_IPSERVER = 100;
+
+}