I was wondering if it is possible to have an android phone send a command to arduino over bluetooth which than triggers a pulse to open/shut a valve for the duration of the button-press (on the phone).
Yes, it is. If you program the android appropriately.
I want to use this to turn on multiple lights.
Lights controlled by a valve?
I checked the web and I think it should be possible using the ADK board and a Bluetooth shield.
Well, you know the web is always right. In this case, it really is.
What would be the best way to do it
Without any mistakes.