WifiManager: Add new API to restore old backup data

Add a new API to restore the old backups containing the raw data from
wpa_supplicant.conf & ipconfig.txt files.
This helps clean up all the existing wifi configuration parsing
logic from SettingsBackupAgent.

BUG: 29075035
Change-Id: I8c7b93ef7c9e2decc8e4f9e5d7038939cf0ef196