Revert "Add NATT keepalive resources and methods into IpSecService"

This reverts commit 2445227f.

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: Ibff278a6eee666cd85dba81c2bed94d568679b02