[WifiSettings] Refactor Wi-Fi required info

Instead of using an invisible TextView in the layout itself, make the
required info a list header. This paves way for using the Setup
Wizard library by including all the required contents in the ListView
itself.

The required info is not shown since Lollipop, but can be tested with
    adb shell am start -a "com.android.net.wifi.SETUP_WIFI_NETWORK" \
            --es theme material_light \
            --ez useImmersiveMode true \
            --ez wifi_show_wifi_required_info true

Bug: 19698532
Change-Id: I5a7da726b2f49af7dd755b41c73e1a1b4349699c