In order to send an Android push message from a server, two pieces of information are required:
The common wisdom says that the sender id should be apparent in my project page, but I can't seem to find it - any idea where is it?
After 20 minutes of agony, I found the problem - I was in the API section of the console (where the server keys are), and I needed the main console page (where the project id / sender id is).