During flight from one country to another, can I use my hand held radio during flight to listen to ATC?
This actually varies from country to country. You can use it in the US (but I cant promise people wont look at you funny). In the UK its actually illegal to do so you can see a full discussion here on that topic. If you intend on doing this I would advise checking the local laws on what you can and cant do.
I don't know if you are "authorized" to... but I can tell you that it likely won't work.
I've tried a handheld aviation radio in a commercial cabin, and was unable to hear the ATC side of transmissions at all. This was likely due to having a very short antenna inside a large metal cylinder, a long way from the source of the transmission.
It was nearly impossible to follow the communications when I could only hear my flight crew's side of the conversation.