Advice on saving in-app purchase tokens on Android

I would like to store details of which in-app purchases a user has purchased on my Android app and would like advice on what solutions others use to save this data.

I believe the simplest method is to save such data as a shared object. I have no problem doing this as already save some config data in the app using a shared object. I assume this is not very secure - however I have no idea myself where to find the shared objects on my android device - where are they and how can I access them? I am a newbie when it comes to Android though.

If there are better options, I would appreciate any examples. I have read that this data can be saved to my own server but for this solution, I am not sure what unique identifier would be used to link the data to the user or device. Also what happens if the device is offline?