Add KeepalivePacketDataTest

Add test for constructor of KeepalivePacketData and
InvalidPacketException

Bug: 151109466
Test: atest CtsNetTestCasesLatestSdk:android.net.KeepalivePacketDataTest
      on both Q and R devices
Change-Id: Ic9db4b9b83ff96c87f41c5f381763c3ee8dd27c8
Merged-In: Ic9db4b9b83ff96c87f41c5f381763c3ee8dd27c8