Improve UX of Wi-Fi QR code scanner enrollee flow

1. Do not start WifiSettings after enrollee success
2. Return WifiConfiguration to the calling object after enrollee success
3. In WifiDialog, do not show scan button if Save button is not visible to users
4. Fix scanner button may not respond problem by setting listener for scanner
   button of ssid field and password field
5. If SSID is assigned, only allow to enrollee for the network of assigned SSID

Bug: 124416152
Test: manual test
Change-Id: Ic5241c4767eaa8ea01782817d8302d8b0572ab36

src/com/android/settings/wifi/dpp/WifiNetworkConfig.java

@@ -228,7 +228,7 @@ public class WifiNetworkConfig {
     /**
      * This is a simplified method from {@code WifiConfigController.getConfig()}
      */
-    private WifiConfiguration getWifiConfigurationOrNull() {
+    WifiConfiguration getWifiConfigurationOrNull() {
         if (!isValidConfig(this)) {
             return null;
         }