Fix "Add Network" show last selected WiFi AP Dialog

When user taps Wi-Fi add network button, the pop-up dialog is not empty (it shows last selected WiFi AP Dialog).
The above is observed after the following steps:
- Launch Settings application
- Turn on Wi-Fi
- Select any Wi-Fi Access Point which is not connected previously and change to portrait mode
- Press Cancel from the pop-up and select Add Network icon in menu

Because the "Rotation" operation will cause WifiDialog re-creating and the current settings
of selected Wi-Fi Access Point is saved before WifiDialog destroyed.
Then, the saved Wi-Fi Access Point is displayed when user tap on "Add Network".

It need to clear the saved Wi-Fi Access Point when WifiDialog re-creating.

Change-Id: I3e93943e5bee405329e309c3ec88a35ee9cafc43