handleNewMnemonic(mnemonic) {
    if (!this._getRaw("randomizeMnemonicOnStart", true)) {
      this.set("server.mnemonic", mnemonic);
    }
  }
Called when a new mnemonic is read back from the underlying chain, will
persist this new mnemonic if it differs from the one stored and if
randomizeMnemonicOnStart is false.