Plumb NattKeepalive from HAL to DataConnection

Connect NetworkAgent's Packet Keepalive signals
to the telephony HAL via DataConnection and the
CommandsInterface.

Add DcKeepaliveTracker to DcNetworkAgent to track
incoming keepalive indications and handle status
reporting to the NetworkAgent

Bug: 38350389
Test: verified with sl4a, additional tests pending
Change-Id: If436b9240b192ed41f2dd0d05aca1ae36dc5e57f