When it comes to bringing your own food and drinks into a movie theatre it is all about discretion and respect. It is going to be against the rules at nearly any cinema to bring outside food and drink inside, however if you are discrete and respectful about doing so you may not have any problems whatsoever. The food you bring inside should not be food that it is not easy to consume, for instance you should not be bringing a whole meal that requires a fork or a spoon into a movie theatre with you.
You should also not be bringing in things that are going to give off an excessive smell or make excessive noise while consuming. Technically you are expected to partake of whatever the cinema has to offer but if you are discrete and respectful bringing a few outside things in should be fine.