Rework KeepalivePacketData for Compatibility with Cell

KeepalivePacketData currently mixes multiple concepts: the
list of parameters that are used to generate a keepalive
packet, the keepalive packet itself, and the parameters that
are needed to send a keepalive packet over an ethernet link.

The KeepalivePacketData is now a parcelable that can be used
generically by any NetworkAgent, regardless of how that Agent
fulfills its duty to initiate and maintain a keepalive session.

Bug: 69063212
Test: verified with SL4A, additional tests pending
Change-Id: I23dc4827ae729583356a8ff0f02e39a2ad2b81f5