Install apps silently, with granted INSTALL_PACKAGES permission

I am trying to silently install apk into the system. My app is located in /system/app and successfully granted permission "android.permission.INSTALL_PACKAGES"

However I can't find anywhere how to use this permission. I tried to copy files to /data/app and had no success. Also I tried using this code

    Intent intent = new Intent(Intent.ACTION_VIEW);
    intent.setDataAndType(
            Uri.parse("file:///sdcard/app.apk"),
            "application/vnd.android.package-archive");
    startActivity(intent);

But this code opens standard installation dialog. How can I install app silently without root with granted android.permission.INSTALL_PACKAGES?

PS I am writing an app that will install many apks from folder into the system on the first start (replace Setup Wizard). I need it to make firmware lighter.

If you think that I am writing a virus: All programs are installed into /data/app. Permission Install_packages can only be granted to system-level programs located in /system/app or signed with the system key. So virus can't get there.

As said http://www.mail-archive.com/android-porting@googlegroups.com/msg06281.html apps CAN be silent installed if they have install_packages permission. Moreover you don't need Install_packages permission to install packages not silently. Plus http://www.androidzoom.com/android_applications/tools/silent-installer_wgqi.html