[Android]Flutter页面销毁时，无法dispose PlatformView

## Steps to Reproduce
**A small application to reproduce the bug(最小化可复现的demo)**
将exmaple中`flutterPage`对应的页面改成`NativeViewExample`后，点击`open flutter page`按钮打开NativeView页面，再退出，重复打开退出操作。会发现页面越来越卡，内存占用越来越大。

**Flutter Boost Version**
4.2.2

**Target Platform:**
Android

## Reason
页面退出的时候，会在onDestroy中触发perfromDetach方法，其中会调用`PlatformViewsController`的detach方法
``` java
public void detach() {
if (platformViewsChannel != null) {
platformViewsChannel.setPlatformViewsHandler(null);
}
destroyOverlaySurfaces();
platformViewsChannel = null;
context = null;
textureRegistry = null;
}
```
将PlatformViewsHandler设置为null。这个就导致`PlatformViewsChannel`中再接收到Flutter层传来的dispose消息时，因为handler为null，直接return了，所以无法dispose那些PlatformView。
``` java
private final MethodChannel.MethodCallHandler parsingHandler =
new MethodChannel.MethodCallHandler() {
@Override
public void onMethodCall(@NonNull MethodCall call, @NonNull MethodChannel.Result result) {
// If there is no handler to respond to this message then we don't need to
// parse it. Return.
if (handler == null) {
return;
}
......
}
};
```

## Fix
在FlutterBoostActivity和FlutterBoostFragment的onDestroy方法中，使用反射，调用`PlatformViewController.flushAllViews`方法。
修复后，虽然能够dispose了并且没有了Java层的内存泄漏，但发现Native Memory仍会上涨，感觉是Surface相关的内存没有释放，不知道是哪的原因。

Trace the Android lifecycle from FlutterBoostActivity and FlutterBoostFragment onDestroy through PlatformViewsController.detach and PlatformViewsChannel handling. Review the reported flushAllViews workaround and reproduce repeated NativeViewExample open/close cycles. Done means PlatformViews are disposed and both Java and native memory, including Surface-related memory, no longer grow.

Written by the indexing model from the issue text.