BpfCoordinator: ignore stopping event monitoring if never started

Ignore stopping monitoring if the monitor has never started
for a given IpServer.

Also fix the related bug in
BpfCoordinator#testStartStopConntrackMonitoring.

Bug: 250552545
Test: atest BpfCoordinatorTest
Change-Id: I7ac71f0ed52bf52244aadaa3c90ca8df239873e4