Add removeAll logic for legacy keystore when

WifiBlobStore is not supported.

Bug: 365543479
Flag: android.net.wifi.flags.wifi_keystore_remove_all_api
Test: atest WifiKeystoreTest
Test: Manual test - add several certificates to legacy
      keystore and verify that they are cleared by
      calling removeAll
Change-Id: Ie8dc210b452729fe450dacd9ad4cc2dd60af7f57