Dialer: convert VoicemailSecretCodeActivity to androidx

* fixes Fragment$InstantiationException on launch. PreferenceActivity
  cannot instantiate PreferenceFragmentCompat due to it being a
  androidx.fragment.app.Fragment instead of the expected
  android.app.Fragment.
* also fix double listing of preferences in override screen.

Change-Id: Ibd9b1fe064cff23b81aca592bdde2b41c200b784