Implement getConfig() method to get WifiConfiguration

Make a duplicated function of WifiConfigController.getConfig(). This
function could get a new WifiConfiguration from input AccessPoint or
ScanResult.
Should be removed if there is standard one in framework.

Bug: 120827021
Test: RunSettingsRoboTests

Change-Id: Ia5981e8d41f434c097b464a2bfe18e1356f7b087