Settings: migrate usage of @hide NetworkSuggestion APIs

Settings uses @hide WifiNetworkSuggestion.
wifiConfiguration/passpointConfiguration.
Migrate to @SystemApi getters.

Removed call to PasspointConfiguration.validate()
as it is an @hide API, instead
WifiManager.addOrUpdatePasspointConfiguration()
will perform the required validation.

Bug: 138801922
Test: make RunSettingsRoboTests -j40
Change-Id: I748010b66634b1d24e89a175917492f638267cb1