[Wi-Fi] Add WifiSavedConfigUtils#getAllConfigsCount

For BaseSearchIndexProvider#getNonIndexableKeys to get saved
networks count in WifiSettings2.

Bug: 146823365
Test: manual
      In Settings APP, search 'Saved networks' when there is
      a saved Wi-Fi network and when there is no saved Wi-Fi network.
Change-Id: I8f16d2b85ab19facc5c43d3fa85e885e53b7c53d