Try reading some old dragnet scripts for a feel of what it used to be like.
I think they used "citizens" sometimes in the bits before the show talking about the city and more often they used "people" but also used "the public" and "man" and "woman".
I don't recall "civilian" being used.
There's a blog from 2009 about this
It looks like the international definition of civilians includes police officers. ("A civilian under international humanitarian law (also known as the laws of war) is a person who is not a member of his or her countryâ(TM)s armed forces or other militia. Civilians are distinct from combatants." )
I've been unable to google when the police started using "civilian" widely. Most states still advertise for "civilian police force" jobs. So the police are explicitly civilian in the state's eyes.