Move IpUtils to frameworks/libs/net

The utility classes are useful for mainline modules and their tests, for
example IpClientIntegrationTest.

Bug: 168868607
Test: m && atest NetworkStaticLibTests
Change-Id: If2b1613aa18a7990391e2d31cc2951ca93f1cf3c
diff --git a/Tethering/tests/privileged/src/android/net/ip/DadProxyTest.java b/Tethering/tests/privileged/src/android/net/ip/DadProxyTest.java
index 747d3e8..95e36fa 100644
--- a/Tethering/tests/privileged/src/android/net/ip/DadProxyTest.java
+++ b/Tethering/tests/privileged/src/android/net/ip/DadProxyTest.java
@@ -33,7 +33,6 @@
 import android.net.TestNetworkInterface;
 import android.net.TestNetworkManager;
 import android.net.util.InterfaceParams;
-import android.net.util.IpUtils;
 import android.net.util.TetheringUtils;
 import android.os.Handler;
 import android.os.HandlerThread;
@@ -46,6 +45,7 @@
 import androidx.test.filters.SmallTest;
 import androidx.test.runner.AndroidJUnit4;
 
+import com.android.net.module.util.IpUtils;
 import com.android.testutils.TapPacketReader;
 
 import org.junit.After;