This can be solved (with root privileges)
goto /system/usr/keylayout/ and modify cyttsp_button.kl, Generic.kl and synaptics_dsx.kl changing:
key 139 MENU VIRTUAL
key 158 BACK VIRTUAL
into:
key 139 BACK VIRTUAL
key 158 MENU VIRTUAL
Then reboot. Tried on my mi5s and...