If you can pre-verify the need for your app by getting people to sign up for a mailing list or otherwise, then go for the app, otherwise get a job.
I mean: Which is more valuable for interviewers: having couple of working and deployed projects or be good at doing coding tasks? (I have 3 year programming experience)
Case in point: https://twitter.com/mxcl/status/608682016205344768?lang=en