Only show Sign In button for connected network

The Sign In button may appeared in details page for disconnected network
if current network is Captive Portal network. Should hide the button
for disconnected network.

Bug: 129514576
Test: make RunSettingsRoboTests -j32 ROBOTEST_FILTER=com.android.settings.wifi.details.WifiDetailPreferenceControllerTest
Test: manual test
Change-Id: I8b4f79e7ba731ce83c49576fffd3e78a27598be3