If user click connect button and then receive timeout event after leave
detail page, the NPE happened. Ignore the timeout event if activity
already gone.
Bug: 131141866
Test: manual test
Test: make RunSettingsRoboTests -j32 ROBOTEST_FILTER=com.android.settings.wifi.details.WifiDetailPreferenceControllerTest
Change-Id: Ic22c22027386d4a6de5d693eafcd9cd463ed3415