To build the app run yarn build. The dist folder will be generated. Inside the ** dist** folder there will be an index.html and a folder called android_resources. Copy the content of the android_resouces folder and paste it into the dist folder, so that it's next to index.html.
This has to be done because Android needs src paths to start with /android_resources/, but eventually this step will be automated.
Now open the Android project files and copy the content of the dist folder the src/main/assets/.