Fix Wi-Fi DPP fragment back stack problems

1. When a Wi-Fi network connected and receiving
   intent android.settings.PROCESS_WIFI_EASY_CONNECT_QR_CODE,
   show WifiDppAddDeviceFragment from WifiDppConfiguratorActivity
   instead of WifiNetworkListFragment

2. Only handle intent when savedInstanceState is null

3. To show a fragment
   I. do nothing when a fragment is already visible on screen,
   II. pop back stack if it's in back stack, we can do this because
   current fragment transactions are arranged in an order

Bug: 124194531
Test: atest WifiQrCodeTest
      atest WifiDppConfiguratorActivityTest
      atest WifiDppEnrolleeActivityTest
      atest WifiDppQrCodeGeneratorFragmentTest
      atest WifiDppQrCodeScannerFragmentTest
      atest WifiNetworkListFragmentTest
      atest WifiDppChooseSavedWifiNetworkFragmentTest

Change-Id: I1d44eeb3bdc7bfb2ec2e4d81efaab64b066651ad