Use APKPure App
Get FastDormancy Toggle for i9300 old version APK for Android
Toggles FastDormancy on "some" Samsung devices.
This app can toggle fastdormancy for i9300 using the method described in http://www.androidnz.net/2012/06/at-your-own-risk-how-to-disable-fast.html
it is only tested on Samsung Galaxy S3 (i9300).
Your device needs to be rooted for this app to work.