Support constant extension for android and iOS #2369

#In react-native, we have getConstants to expose a map to js:

  @Override
  public Map<String, Object> getConstants() {
    final Map<String, Object> constants = new HashMap<>();
    constants.put(DURATION_SHORT_KEY, Toast.LENGTH_SHORT);
    constants.put(DURATION_LONG_KEY, Toast.LENGTH_LONG);
    return constants;
  }

So that js can access those constants directly without a promise. However in weex, registerModule can only do this with a callback. This is really useful if the common configs are on native side.

For more details, please refer: http://facebook.github.io/react-native/docs/native-modules-android.html