Skip stop if keepalive is already in stopping state

In current design, crash has been generated when stop function
has been re-entered to catch unexpected behavior. However,
it is possible to re-enter stop function if the network
disconnection occurs after stopping.

Thus, skip stop if keepalive is already in stopping state.

Test: atest ConnectivityServiceTest#testNattSocketKeepalives \
      --rerun-until-failure 60000
Bug: 167332570
Change-Id: Ic7068ad3dc990e957c37b8d87d48ebb6469b101f