I'd have to say web development. When I say that I mean these skills specifically:
SQL (I recommend PostgreSQL), one of Python/Ruby/Scala, HTML5, JavaScript, CSS, and one of the popular JS front end client libraries, like Ember or AngularJS.
I'm working on that skillset myself right now, having already learned android development and at least given iOS a spin.
As for Android, be prepared to have the Dev team changing the damn API all the time.
For iOS, you're gonna pay up front just for the privilege to develop.
For both Android and iOS, you're looking at using a monolithic app store as your primary distribution mechanism for whatever software you write and being cheifly at the mercy of Google and Apple. Sure, there's other app stores... They're not enough to put dinner on the table.
If you're especially looking to be able to develop your own MVPs and launch something, just learn the web side yourself, develop that, make an API, and expose it. If your app is good enough people will just write mobile clients for it and they'll sell them on the app stores...