services.net: Add a static library needed by wifi module

Add a new static library that compiles against system_current which
includes all the parts needed by the wifi mainline module.

Also,
a) Refactored TcpKeepalivePacketData to use public APIs. The parcel
read/write methods in the base class are @hide and they're not used by
the other child class (NatKeepalivePacketData). So, remove the @hide
method from base class and use it direcly in the child class.

Bug: 145825329
Test: Device boots up & connects to wifi networks.
Change-Id: Ifde69b579cfe5b813766f676acb10e436e64a44c
Merged-In: Ifde69b579cfe5b813766f676acb10e436e64a44c