System Notification - LG G3 mobile is infected - update now

System Notification - LG G3 mobile  is infected - update now was message I got while  on internet. Informs the following steps to do:

Step 1. Tap the button and install the APK file

Step 2. Open the application to start the antivirus scan now.

I checked Norton Mobile activity log and it informs that the device is always protected, App Advisor: No problem apps found.

Why did I get System notification that my mobile is infected when Norton Mobile activity log says protected? Norton scans is set to scan daily.

what is an APK file?

Please update what steps to take to make sure my mobile is not infected? How can I make sure that my mobile is not infected and it is secure to connect to a pc?

Looking forward for the input so that I can connect my mobile to my pc

Thank You

Rao

PS: Product: Norton 360 multi device, Device Name LG G3