code: boolean clearAll() {
          SharedPreferences.Editor editor = sharedPreferences.edit();
          Map<String, ?> all = sharedPreferences.getAll();
          //noinspection Convert2streamapi
          for (String key : all.keySet()) {
              editor.remove(key);
          }
          return editor.commit();
      }
docs: Clear all entries in internal shared preferences file.
  
  @return Returns true if the new values were successfully written