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\"
Its possible to do silent install on Android 6 and above. Using the function supplied in the answer by Boris Treukhov, ignore everything else in the post, root is not required either.
Install your app as device admin, you can have full kiosk mode with silent install of updates in the background.