WifiManager/WifiScanner: Misc API cleanup

- Protected getCurrentNetwork() with a privileged
    permission
- Undeprecated restoreSupplicantBackupData: we
    will still need to support restoring old backup
    data for the foreseeable future. What is not
    supported is getting backup data in the old
    supplicant format.
- Cleanup annotation for WifiScanner: the entire class
    is @SystemApi, no need to mark the individual
    methods @SystemApi

Bug: 144925765
Test: compiles
Change-Id: Ie3852af04efa88def4f52499e7c802f0adb747e8