[KA09] add keepalive offload permission to shell

This change adds PACKET_KEEPALIVE_OFFLOAD to shell, which allows
shell or code with shell permission identity to use privileged
tcp keepalive offload API.

Bug: 114151147
Test: -atest ConnectivityManagerTest#testCreateTcpKeepalive
      -build, flash, boot

Change-Id: Ib6660a5eaa72f83042596481452be4d415383f02
Merged-in: Ib6660a5eaa72f83042596481452be4d415383f02