I tried to make the arrows invisible by hacking /usr/share/gnome-shell/theme/ubuntu.css
file.
Disclaimer: playing with shell files is very dangerous, small mistakes will stop access to GNOME desktop session. Only tty will work. I have several times reinstalled Ubuntu in such cases. although I have successfully achieved the required. Do this on your own risk.
found this part,
.screen-shield-arrows Gjs_Arrow {
color: white;
width: 80px;
height: 48px;
-arrow-thickness: 12px;
-arrow-shadow: 0 1px 1px rgba(0, 0, 0, 0.4); }
and made it to
.screen-shield-arrows Gjs_Arrow {
color: white;
width: 0px;
height:0px;
-arrow-thickness: 12px;
-arrow-shadow: 0 1px 1px rgba(0, 0, 0, 0.4); }
hack: width & height fields made to 0px

Observe that at every 7th second, arrows should be visible..
now no more with the above edits in /usr/share/gnome-shell/theme/ubuntu.css
file.