Revert "[KA11] Verify fd ownership and allocate resource for NattKeepalive"

This reverts commit cecd4a3d.

Reason for revert: Adds dependency between IpSecService and
                   ConnectivityService may lead to future deadlock
                   problems. Uses a simpler approach instead,
                   hence the solution is not needed.
                   See aosp/954040.

Change-Id: If6d537a39595cf132d3ed81d4eaac6700f5f0ab3