Automatically direct the user to the captive portal in Wi-Fi Slice

Save the SSID when the user clicks the network, and then automatically
start login page when the network gets the captive portal capability.

Fixes: 128056349
Test: make RunSettingsRoboTests -j ROBOTEST_FILTER=com.android.settings.wifi
Change-Id: Ia25241a8243d7d6aae604f341b512350404d9fd1