已经pause的页面destroy后没有调用dispose

## Steps to Reproduce
**A small application to reproduce the bug(最小化可复现的demo)**

1. 把demo的MainActivity启动模式改为singleTask
2. NativePageActivity 的点击事件处理改为打开MainActivity
3. simple_page_widgets.dart中_FlutterRouteWidgetState增加initState和dispose方法，打印日志
4. 安装demo，依次open flutter page->open native page->open native page
5. 查看日志，只打印了initState,没有打印dispose，而这个时候flutter页面其实已经不在了
6. 重新打开flutter页面发现打印日志initState dispose

**Flutter Boost Version:** 1.12.13+3
**Target Platform:** Android
**Target OS version/browser:** 8.1.0
**Devices:** PIXEL

## Logs
add your crash log or something else.

```
// 1-5步骤打印日志
I/flutter (25896): flutterboost:initState

// 6步骤打印日志
I/flutter (25896): flutterboost:initState
I/flutter (25896): flutterboost:dispose
```

Start by running the reproduction sequence on Android 8.1 with MainActivity in singleTask mode, then inspect simple_page_widgets.dart and the NativePageActivity click handling. Trace the Flutter page lifecycle across the native-page transitions and compare the logged initState and dispose calls. Done means the affected page is disposed when it is no longer present, with the lifecycle behavior covered by a reproducible check.

Written by the indexing model from the issue text.