Add keepalive related methods and fields to system APIs

Add KeepalivePacketData to system API for mainline support.
Also, remove InvalidPacketException class from SocketKeepalive
and create a new InvalidPacketException class in android.net

Bug: 139268426
Bug: 135998869
Bug: 138306002
Test: atest FrameworksNetTests
      atest NetworkStackTests
      atest FrameworksTelephonyTests
      ./frameworks/opt/net/wifi/tests/wifitests/runtests.sh
      atest android.net.cts
      atest android.net.wifi.cts
      atest android.telephony.cts

Change-Id: I2d982e8abb5cb6b4c74a20483550b18cf814320d
Merged-In: I2d982e8abb5cb6b4c74a20483550b18cf814320d
diff --git a/core/java/android/net/NattKeepalivePacketData.java b/core/java/android/net/NattKeepalivePacketData.java
index a77c244..3fb52f1 100644
--- a/core/java/android/net/NattKeepalivePacketData.java
+++ b/core/java/android/net/NattKeepalivePacketData.java
@@ -19,7 +19,6 @@
 import static android.net.SocketKeepalive.ERROR_INVALID_IP_ADDRESS;
 import static android.net.SocketKeepalive.ERROR_INVALID_PORT;
 
-import android.net.SocketKeepalive.InvalidPacketException;
 import android.net.util.IpUtils;
 import android.os.Parcel;
 import android.os.Parcelable;
@@ -32,6 +31,9 @@
 
 /** @hide */
 public final class NattKeepalivePacketData extends KeepalivePacketData implements Parcelable {
+    private static final int IPV4_HEADER_LENGTH = 20;
+    private static final int UDP_HEADER_LENGTH = 8;
+
     // This should only be constructed via static factory methods, such as
     // nattKeepalivePacket
     private NattKeepalivePacketData(InetAddress srcAddress, int srcPort,