Add a removeAll method to WifiKeystore.

Bug: 365543479
Flag: android.net.wifi.flags.wifi_keystore_remove_all_api
Test: atest FrameworksWifiNonUpdatableApiTests
Test: Manual test - add several entries to WifiKeystore
      and check that calling removeAll removes them
      from the database
Change-Id: Ibf8e8f8b9588cf64f2d3e746a5f4d5211f16144b