[KA02] internal cleanup and refactor for SocketKeepalive

In previous change, the new SocketKeepalive API was exported.
But internally, old PacketKeepalive names and structures are
still used.

This change rename them properly for code consistency and also
refactor KeepalivePacketData to support different types of
KeepalivePacketData.

(clean cherry-pick from aosp/860394)

Bug: 114151147
Test: 1. atest FrameworksNetTests
      2. atest FrameworksWifiTests
      3. atest FrameworksTelephonyTests

Change-Id: Ia9917d12987e91e87e34ffb3f126e7bc7c9c187e