Is there a way to "reset" idle time when unplugging laptop?

Reset idle time when unplugging power, or reset idle time while you are on AC?

Even if you'd manage to create an action on unplugging (you can), the question is wether you would be in time to prevent suspending.

A more reliable trick is to prevent getting idle time pass the idle-time threshold at all, but only if you are on AC.

If you are on X, how can we do that?

Run a tiny daemon-like background script that - only when you are on AC and idle for more than (let's say) a minute - simulates a key press Control. That's a key that does nothing if you are not actually working on the laptop, only resets idle time.
Then if you unplug, the virtual keypress is skipped and the laptop will do whatever you set it to do after x time.
Below an example of such a script. In a comment, you mentioned you are using Ubuntu 18.04, Gnome, which means you are using Mutter (X), so you can use the tools in the script below.

How to use?

1. Make sure xprintidle, xdotool and acpi are installed: sudo apt install xdotool acpi xprintidle
2. Copy the script into an empty file.
3. Test-run it from a terminal: python3 /path/to/script. If all works fine, add it to your startup commands.

The script

#!/usr/bin/env python3
import subprocess
import gi
gi.require_version("Gtk", "3.0")
from gi.repository import GLib, Gtk

def test(arg):
    # let's see what is the current idle time
    idletime = int(subprocess.check_output("xprintidle").decode("utf-8"))
    # and if we are plugged in
    onpower = "on-line" in subprocess.check_output(
        ["acpi", "-a"]
    ).decode("utf-8")
    # if idle for > 60 seconds AND on power, tap Ctrl to reset idle
    if all([idletime/1000 > 60, onpower]):
        subprocess.Popen(
            ["xdotool", "key", "Ctrl"]
        )
    # keep it running please
    return True

GLib.timeout_add_seconds(15, test, None)

Gtk.main()