[ohos]: 鸿蒙端没有unset方法

### 请描述遇到的问题，以及您所期望的正确的结果

目前只有setup方法初始化引擎，没有办法销毁引擎，一些情况如登录状态变更需要销毁引擎重新创建

### 请说明如何操作会遇到上述问题

_No response_

### 在下面填入关键复现代码

_No response_

### 复现的平台

Android

### Flutter SDK版本

鸿蒙fluttersdk dev分支

### FlutterBoost版本

main分支

### 是否延迟初始化FlutterBoost

No

### 解决方案

The issue does not name any files, tests, or entry points. Start by locating the OHOS setup implementation and the FlutterBoost engine lifecycle, then determine how an unset operation should destroy the engine and support later recreation. Done means the lifecycle behavior is defined and verified for the reported login-state change case.

Written by the indexing model from the issue text.