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