[Wi-Fi] Support hidden SSID Wi-Fi networks for Wi-Fi QR code scanner & generator

WifiTracker#getAccessPoints() only contains a hidden SSID Wi-Fi
network if it's a saved Wi-Fi network. We can't check if a hidden
SSID Wi-Fi network is reachable inadvance. That is the difference
after a user scanned a QR code of hidden SSID Wi-Fi network.

Bug: 135892509
Test: make RunSettingsRoboTests ROBOTEST_FILTER=WifiDppUtilsTest
      manual test:
      Click the Share button of a connected hidden SSID Wi-Fi network
      and save the QR code image. Use Wi-Fi QR code scanner in Wi-Fi
      Settings to scan the saved QR code image.

Change-Id: I9056622e6dadb04e5642ae1c50fb4d227a78d26d