SUCCESS!
First thank you @obj for help and guidance. I really appreciate it.
Here is what happenned:
it was an oversight that I missed rtlsdr in groups list.
Just after posting my last post, I reviewed the groups list again and caught rtlsdr.
[abcd@fedora ~]$ sudo usermod -a -G rtlsdr dump1090
[abcd@fedora ~]$ sudo nano /lib/systemd/system/dump1090-fa.service
#UN-COMMENTED LINE "User=dump1090"
[abcd@fedora ~]$ sudo systemctl daemon-reload
[abcd@fedora ~]$ sudo systemctl restart dump1090-fa
[abcd@fedora ~]$ sudo systemctl status dump1090-fa -l
● dump1090-fa.service - dump1090 ADS-B receiver (FlightAware customization)
Loaded: loaded (/usr/lib/systemd/system/dump1090-fa.service; enabled; vendor>
Active: active (running) since Tue 2018-08-28 12:44:30 EDT; 3s ago
Docs: https://flightaware.com/adsb/piaware/
Main PID: 1760 (dump1090-fa)
Tasks: 3 (limit: 2359)
Memory: 5.1M
CGroup: /system.slice/dump1090-fa.service
└─1760 /usr/bin/dump1090-fa --device-index 0 --gain -10 --ppm 0 --ne>
Aug 28 12:44:30 fedora systemd[1]: Started dump1090 ADS-B receiver (FlightAware>
Aug 28 12:44:30 fedora dump1090-fa[1760]: Tue Aug 28 12:44:30 2018 EDT dump109>
Aug 28 12:44:31 fedora dump1090-fa[1760]: rtlsdr: using device #0: Generic RTL2>
Aug 28 12:44:31 fedora dump1090-fa[1760]: Detached kernel driver
Aug 28 12:44:32 fedora dump1090-fa[1760]: Found Rafael Micro R820T tuner
Aug 28 12:44:32 fedora dump1090-fa[1760]: rtlsdr: enabling tuner AGC