Wifi: Maintain configKey generation logic same in backup agent

AutoJoinController uses configKey to distinguish networks. It compares
configKeys of saved network with recent ScanResult. WifiConfiguration
has two different APIs to generate configKey based on input variables.
Framework should use same logic to generate configkey even though input
differs. This fix ensures the parity in backup agent.

Change-Id: I6ee57efe1d346e50a434bce90ec8224f5203712c