The idea is to send the same commands from the Kodi app as the custom Android app does to the Arduino controlled relays. Before the movie starts, the lights go off. I'll skip the details of that code, but point to a few useful pages to get started. It's simple.
The Kodi Add-on documentation is good. To get started, you need at least two files: The addon.xml configuration, and your Python script, e.g. myaddon.py. These have to be in a directory on the format script.name (more in the linked documentation) and zip-ed in a ZIP file which does not use use compression, as seen below. This zip-file can now be copied to the NAS, and installed from Kodi.
zip -0 -r myaddon.zip script.myaddon
One gottcha is that the addon.xml file cannot contain a final new-line. At least some people have reported that causing a install error.