It's quite simple. You're at a university: the lecturer is there to deliver a lecture to the hall not to teach you. You're not at school anymore, the lecturer isn't going to (or at least shouldn't) tell you off for how much attention (or lack thereof) that they perceive you are paying.
You (or your government) are paying (for you) to be there so as long as you're not disrupting other people, what you do and don't do is entirely up to you. If such an innocuous thing upsets the lecturer, that's on them not on you.
If you're in a more intimate setting like a tutorial where you're actually being taught, just ask the lecturer if they mind.