updateAlpha method
- double alpha
更新透明度
Android独有
Implementation
Future<bool> updateAlpha(double alpha) async {
  bool ret = await BMFMapDispatcherFactory.instance.markerDispatcher
      .updateMarkerMember(this.methodChannel, {
    'id': this.Id,
    'member': 'alpha',
    'value': alpha,
  });
  if (ret) {
    this.alpha = alpha;
  }
  return ret;
}