Android FlutterBoostFragment 白屏问题

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

1. 在 FlutterBoostFragment 中打开一个透明的 FlutterBoostActivity
2. 按下电源键锁屏，再按下电源键点亮屏幕，此时可以看到 Frgment 变成白色了
3, 退出透明的 FlutterBoostActivity, FlutterBoostFragment 页面恢复显示

此问题概率性出现，出现概率超过50%

**Flutter Boost Version**
**Target Platform:**
**Target OS version/browser:**
**Devices:**

## Logs
add your crash log or something else.

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Start with the README and reproduce the lock-screen sequence using FlutterBoostFragment and a transparent FlutterBoostActivity. Inspect those entry points around screen resume and rendering, using flutter doctor -v for environment details. Done means the Fragment remains correctly displayed after unlocking and after exiting the transparent activity.

Written by the indexing model from the issue text.