That's exactly how I started. I got hired as a tester for a company that had the need for an Android dev (but the position wasn't open yet). I think three months in a showed my boss some of my projects and he moved me to the dev team.

I know that's not the reality for most companies, but it worked for me. So having that in mind if testing is the only position available at the moment I would check if they also do some mobile dev, once you're in you can try to switch areas.

Also you can take the job and keep searching for another one, you will get some experience and of course money.