实现添加悬浮窗的核心源码代码WindowManagerService#addWindow方法中有以下几个关键：
1.权限检查
mPolicy.checkAddPermission(attrs, appOp);
2.悬浮窗参数调整
mPolicy.adjustWindowParamsLw(win.mAttrs);

查看不同android版本的源码对比

结论：

1. android 4.4(api 19)之前使用TYPE_TOAST 可以显示，但是不能接收触摸事件和按键事件

mPolicy实际上是PhoneWindowManager。
对比checkAddPermission()方法变化结论：

1. android 4.3(api 18) 开始 TYPE_TOAST根本没有做权限检查, 直接break出去了, 最后返回WindowManagerGlobal.ADD_OKAY。这个就是使用TYPE_TOAST不需要权限显示悬浮窗的原因

//android 4.3_r2.1
    public int checkAddPermission(WindowManager.LayoutParams attrs, int[] outAppOp) {
        int type = attrs.type;

        outAppOp[0] = AppOpsManager.OP_NONE;

        if (type < WindowManager.LayoutParams.FIRST_SYSTEM_WINDOW
                || type > WindowManager.LayoutParams.LAST_SYSTEM_WINDOW) {
            return WindowManagerGlobal.ADD_OKAY;
        }
        String permission = null;
        switch (type) {
            case TYPE_TOAST:
                // XXX right now the app process has complete control over
                // this...  should introduce a token to let the system
                // monitor/control what they are doing.
                break;
            case TYPE_DREAM:
            case TYPE_INPUT_METHOD:
            case TYPE_WALLPAPER:
                // The window manager will check these.
                break;
            case TYPE_PHONE:
            case TYPE_PRIORITY_PHONE:
            case TYPE_SYSTEM_ALERT:
            case TYPE_SYSTEM_ERROR:
            case TYPE_SYSTEM_OVERLAY:
                permission = android.Manifest.permission.SYSTEM_ALERT_WINDOW;
                outAppOp[0] = AppOpsManager.OP_SYSTEM_ALERT_WINDOW;
                break;
            default:
                permission = android.Manifest.permission.INTERNAL_SYSTEM_WINDOW;
        }
        if (permission != null) {
            if (mContext.checkCallingOrSelfPermission(permission)
                    != PackageManager.PERMISSION_GRANTED) {
                return WindowManagerGlobal.ADD_PERMISSION_DENIED;
            }
        }
        return WindowManagerGlobal.ADD_OKAY;
    }

//android 4.4_r1
public int checkAddPermission(WindowManager.LayoutParams attrs, int[] outAppOp) {
        int type = attrs.type;
        outAppOp[0] = AppOpsManager.OP_NONE;

        if (type < WindowManager.LayoutParams.FIRST_SYSTEM_WINDOW
                || type > WindowManager.LayoutParams.LAST_SYSTEM_WINDOW) {
            return WindowManagerGlobal.ADD_OKAY;
        }
        String permission = null;
        switch (type) {
            case TYPE_TOAST:
                // XXX right now the app process has complete control over
                // this...  should introduce a token to let the system
                // monitor/control what they are doing.
                break;
            case TYPE_DREAM:
            case TYPE_INPUT_METHOD:
            case TYPE_WALLPAPER:
            case TYPE_PRIVATE_PRESENTATION:
                // The window manager will check these.
                break;
            case TYPE_PHONE:
            case TYPE_PRIORITY_PHONE:
            case TYPE_SYSTEM_ALERT:
            case TYPE_SYSTEM_ERROR:
            case TYPE_SYSTEM_OVERLAY:
                permission = android.Manifest.permission.SYSTEM_ALERT_WINDOW;
                outAppOp[0] = AppOpsManager.OP_SYSTEM_ALERT_WINDOW;
                break;
            default:
                permission = android.Manifest.permission.INTERNAL_SYSTEM_WINDOW;
        }
        if (permission != null) {
            if (mContext.checkCallingOrSelfPermission(permission)
                    != PackageManager.PERMISSION_GRANTED) {
                return WindowManagerGlobal.ADD_PERMISSION_DENIED;
            }
        }
        return WindowManagerGlobal.ADD_OKAY;
}

对比adjustWindowParamsLw（）方法变化结论：

1. android 4.4(api 19) 之前的TYPE_TOAST不能接收触摸事件和按键事件

//android 4.3_r2.1
    public void adjustWindowParamsLw(WindowManager.LayoutParams attrs) {
        switch (attrs.type) {
            case TYPE_SYSTEM_OVERLAY:
            case TYPE_SECURE_SYSTEM_OVERLAY:
            case TYPE_TOAST:
                // These types of windows can't receive input events.
                attrs.flags |= WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE
                        | WindowManager.LayoutParams.FLAG_NOT_TOUCHABLE;
                attrs.flags &= ~WindowManager.LayoutParams.FLAG_WATCH_OUTSIDE_TOUCH;
                break;
        }
    }

//android 4.4_r1
    public void adjustWindowParamsLw(WindowManager.LayoutParams attrs) {
        switch (attrs.type) {
            case TYPE_SYSTEM_OVERLAY:
            case TYPE_SECURE_SYSTEM_OVERLAY:
                // These types of windows can't receive input events.
                attrs.flags |= WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE
                        | WindowManager.LayoutParams.FLAG_NOT_TOUCHABLE;
                attrs.flags &= ~WindowManager.LayoutParams.FLAG_WATCH_OUTSIDE_TOUCH;
                break;
        }
    }