包括通知类弹窗和选择类弹窗，以下是安卓手机的显示效果
关键代码
DisplayAlert("Alert", "You have been alerted", "OK");
var answer = await DisplayAlert("Question?", "Would you like to play a game", "Yes", "No");
var action = await DisplayActionSheet("ActionSheet: Send to?", "Cancel", null, "Email", "Twitter", "Facebook");