service: make sure the watchdog timer is not restarted while stopping

service: make sure the watchdog timer is not restarted while stopping

A watchdog notification may be handled after the watchdog timer was stopped
while stopping the service. As a result the timer is restarted and the
service may be restarted as well.
The watchdog timestamp is initially set during startup in
service_enter_start_post() and cleared when the timer is stopped. Therefore
it can be used as an indication if the timer should be reset.