FlutterBoostView跳转FlutterBoostActivity后回到FlutterBoostView界面，界面不再渲染

### 复现步骤
Demo上即可复现，如视频

从CountDemo回来后，滑动界面就不生效了（实际生效了，不过没渲染了）。

### 原因
跳转到FlutterBoostActivity后再finish，因为`Activity.onDestroy`的调用并不是立即的，导致FlutterBoostView的onResume等生命周期走完后，这时触发`FlutterBoostActivity.onDestroy`，导致再次`flutterView.detachFromFlutterEngine`，使得将Render绑定的surface重新置为了null：

FlutterActivity的onDestroy中的处理，对于FlutterBoost这种单引擎框架的影响，还是太大了。

Reproduce the issue in the Demo by navigating from FlutterBoostView to FlutterBoostActivity and back through CountDemo. Trace the onResume, finish, onDestroy, and detachFromFlutterEngine lifecycle sequence; done means the FlutterBoostView renders again and scrolling remains visible after returning.

Written by the indexing model from the issue text.