Expose WifiNetworkSuggestion.getWifi/PasspointConfiguration()

These @SystemApis are needed by Settings.

Also modified no-arg constructor for
WifiNetworkSuggestion to conform to the
@NonNull guarantees of getWifiConfiguration().

Bug: 138801922
Test: atest FrameworksWifiApiTests
Change-Id: I5ce60ba376e6976bdcb5a651c991c44579bfef3f