Add util method to generate IPv4-mapped IPv6 address from IPv4 address

Cherry-pick of aosp/2795709 to backport VPN security fix to non-mainline
devices.
Having Merged-In to prevent automerger to udc-mainlie-prod where
frameworks/libs/net does not exist.

Bug: 193031925
Test: TH
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:03eb6f09360de988687c06940b6f13be7e650b34)
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:29e53bd862d7b4c30552f188065e1239e84c401e)
Merged-In: Iaa5d5f31904f8a122cb96f2c3f0e3499a2577664
Change-Id: Iaa5d5f31904f8a122cb96f2c3f0e3499a2577664

common/framework/com/android/net/module/util/InetAddressUtils.java

diff --git a/common/framework/com/android/net/module/util/InetAddressUtils.java b/common/framework/com/android/net/module/util/InetAddressUtils.java
index 40fc59f..4b27a97 100644
--- a/common/framework/com/android/net/module/util/InetAddressUtils.java
+++ b/common/framework/com/android/net/module/util/InetAddressUtils.java
@@ -21,6 +21,7 @@
 import android.util.Log;
 
 
+import java.net.Inet4Address;
 import java.net.Inet6Address;
 import java.net.InetAddress;
 import java.net.UnknownHostException;
@@ -32,6 +33,7 @@
 public class InetAddressUtils {
 
     private static final String TAG = InetAddressUtils.class.getSimpleName();
+    private static final int INET4_ADDR_LENGTH = 4;
     private static final int INET6_ADDR_LENGTH = 16;
 
     /**
@@ -93,5 +95,29 @@
         }
     }
 
+    /**
+     * Create a v4-mapped v6 address from v4 address
+     *
+     * @param v4Addr Inet4Address which is converted to v4-mapped v6 address
+     * @return v4-mapped v6 address
+     */
+    public static Inet6Address v4MappedV6Address(@NonNull final Inet4Address v4Addr) {
+        final byte[] v6AddrBytes = new byte[INET6_ADDR_LENGTH];
+        v6AddrBytes[10] = (byte) 0xFF;
+        v6AddrBytes[11] = (byte) 0xFF;
+        System.arraycopy(v4Addr.getAddress(), 0 /* srcPos */, v6AddrBytes, 12 /* dstPos */,
+                INET4_ADDR_LENGTH);
+        try {
+            // Using Inet6Address.getByAddress since InetAddress.getByAddress converts v4-mapped v6
+            // address to v4 address internally and returns Inet4Address
+            return Inet6Address.getByAddress(null /* host */, v6AddrBytes, -1 /* scope_id */);
+        } catch (UnknownHostException impossible) {
+            // getByAddress throws UnknownHostException when the argument is the invalid length
+            // but INET6_ADDR_LENGTH(16) is the valid length.
+            Log.wtf(TAG, "Failed to generate v4-mapped v6 address from " + v4Addr, impossible);
+            return null;
+        }
+    }
+
     private InetAddressUtils() {}
 }