A modern programming language that makes developers happier.

kotlinlang

I'm not a fan of SparkJava because of its reliance on statics everywhere. If you want to start completely bare bones, I'd go with jetty and select the components you need one by one. It's a bit painful but it will teach you a lot about the whole server stack. If you want to start a bit higher level, use DropWizard, which includes low level frameworks so you can focus on higher level things such as creating JSON API (which is probably the first thing you should learn as an Android developer).