WiFi restricted connection when scan QR code

- Use WifiEntry#canConnect() to determine if the network can be
  connected.

- Add WiFi network but don't connect if WiFi restricted connection.

- See the result screenshot in b/203168943#comment16

Bug: 203168938
Bug: 203168943
Test: manual test
make RunSettingsRoboTests \
  ROBOTEST_FILTER=WifiDppQrCodeScannerFragmentTest

Change-Id: I06d1e832176e540ddd1adce8da46ef8659534ab7