Fix an NPE in WifiSettings.

Under stressful conditions, like the Monkey tests, getActivity() can
return null. We assume this occurs when the fragment has shut down but
asynchronous commands are returning results. This change adds appropriate
null checks but assumes during normal operation the checks are not needed.

Bug: 7152711
Change-Id: I7a3736ccead33738dab1a36e78ae1d8042db6e94