[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.

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

Change-Id: Ia9917d12987e91e87e34ffb3f126e7bc7c9c187e
Merged-In: Ia9917d12987e91e87e34ffb3f126e7bc7c9c187e