使用WindowManager添加Window 基本使用：
WindowManager manager = (WindowManager) getSystemService(Context.WINDOW_ SERVICE);
WindowManager.LayoutParams layoutParams = new WindowManager.LayoutParams(width,height,type,flags,format);
manager.addView(btn, layoutParams);
相关flags和type太多了，详解官方文档developer.android.com/reference/a…
Window 层级
- 应用Window的层级范围1~99
- 子Window的层级范围1000~1999
- 系统Window的层级范围2000~2999
如果是系统Window，需要声明权限
<uses-permission android:name="android.permission.SYSTEM_ALERT_WINDOW"/>
