How to disable "dreams" system service
The short answer to kill whatever is running on Android is by using the Activity Manager, Package Manager, and deal with the Service Interface (see here).

However the DreamManagerService on my Lolipop is very tricky and it will complain it's not running at all (even if it is). So what I did was:


Enable the simple screensaver Clock and configure it to run only
when docked. This makes sure it's working properly.
Execute with root the KillDreams.sh script, which kills everything relevant. As a bonus I run it automatically after every reboot using Apps2SD.

sleep 5
echo Normal procedure for am...
am stopservice dreams
sleep 1
echo Force procedure for am...
am force-stop android.service.dreams
am force-stop android.service.dreams.IDreamManager
sleep 1
echo Stop service...
stop dreams
echo Finished...



And voilà! It worked!
Currently my battery lasts nearly double, compared to having it run in the background.