[Wi-Fi] Fix exception when 'Add device' with a Wi-Fi suggestion network

WifiEntry#getWifiConfiguration() may return null when
WifiEntry#isSaved() is true. Use WifiEntry#canEasyConnect()
which checks if WifiEntry#getWifiConfiguration() returns
a non-null value.

Bug: 153029289
Test: manual
      Connect to a WPA2 Wi-Fi network, click 'Add device'
      in Wi-Fi detail page and see if it crashes.
Change-Id: Ia15503eea795fdd54826699828f02c9ff95dcefa