[KA05] Export keepalive offload api for IpSec Nat-T file descriptor

Adds system api of createSocketKeepalive to take file descriptor,
so privileged apps could use it without the need of IpSecService.

Bug: 114151147
Test: atest FrameworksNetTests
Change-Id: If926c21704b6ed73a0adfcadad732b97b42bacae
7 files changed