FlutterBoostFragment 在 fragment 中显示白屏的问题不完美的解决方案

### boost版本号：

- v3.0-preview.13

### 出现原因：

- 子Fragment的onHiddenChanged在Fragment种无法回调

### 解决方式：

- 在fragment的onHiddenChanged中主动hide/show子fragment

Start with FlutterBoostFragment and the Fragment onHiddenChanged behavior described in the issue, using FlutterBoost v3.0-preview.13 as the reproduction context. Verify when the child fragment becomes hidden or visible and compare that with the reported manual hide/show workaround; done means the white screen no longer occurs without relying on that workaround.

Written by the indexing model from the issue text.