qemu.hw.mainkeys=0
if this doesn't work for you,
try this one :
same thing do this at the end of your build prop.
"ro.show.navigationbar=yes"
(without the quotes)
save changes, before you close.
Reboot.
to disable, just type "no" in stead of yes.
(without the quotes, again.)
hope it works.
or try this :
https://apk-dl.com/[root]-soft-buttons/net.alviano.android.softbuttons
install as apk. this will enable and disable your virtual keys, no build prop editing, this app will do it for you. always reboot after each time you use this app.
hope that helps.
