code: private String getOldToken(Context appContext) {
          String jsonData = preferenceProvider.get(appContext).getString(String.format(REGISTRAR_PREFERENCE_TEMPLATE, senderId), "");
          if (jsonData.isEmpty()) {
              return "";
          }
          
          JsonObject jsonedPreferences = new JsonParser().parse(jsonData).getAsJsonObject();
          try {
              return jsonedPreferences.get("deviceToken").getAsString();
          } catch (Exception ignore) {
              //There was something wrong with the deviceToken or the jsonPreferences.  
              //This probably means that there isn't an oldToken.  Let's log and return an empty String
              Log.w(TAG, ignore.getMessage(), ignore);
              return "";
          }
          
      }
docs: Returns the most recently used deviceToken
  @return a deviceToken or an empty string