fix AlertType warning; simplize code

1. animationType is a AlertType, not AlertType *
2. The following two functions (initDropAlertWithTitle:andText:forAlertType:  and  initFadeAlertWithTitle:andText:forAlertType: ) are similar, the only difference is to set the animation type, so I abstract the same code in a private function _initViewWithTitle:andText:forAlertType: