Don't raise if polling manager is running when stopped

Don't raise if polling manager is running when stopped

AsyncProcess.stop() will raise if the process we are trying
to stop is not running. Some functional tests sporadically
fail because in the clean up the polling manager is stopped
without checking if it's still running. Add an except
in InterfacePollingMinimizer.stop() to avoid raising an
exception when the monitor is not running.

Change-Id: Iec8f5039ae5b830836564a1a402ad3e4ba38935d
Closes-bug: #1525856