[KA01] export SocketKeepalive API for NAT-T keepalive

[KA01] export SocketKeepalive API for NAT-T keepalive

The new set of API replace the PacketKeepalive system API
which is only specific for IPsec NAT-T keepalive packet. Allows
applications to request different types of keepalives through
the same set of API.

By using keepalives, applications could request system to
periodically send specific packets on their behalf, using
hardware offload to save battery power.

This change would only address interface change and test part of
original functionality. After all reference of PacketKeepalive
are removed, another change would be submitted to remove old
API.

Bug: 114151147
Test: atest FrameworksNetTests
Change-Id: I3367666810acc44d5ad20e19acd4bf936e8b5d9c