At the very beginning of my CS studies, I played a bit with Android development. You achieve the most learning when you are interested in the stuff you are doing or you have own requirements. My motivation was to write an app, which can speak out incoming notifications. As a text-to-speech engine was already built into Android (e.g. navigation directions), I thought this was a challenging but possible project.
Currently, the project is abandoned, but I fish I find some time to reimplemented it with all the new experiences I gained over the time.
It kind of works and I finished the project ;-)
It is far away from best practice :-(
(This post was written in February 2018. The project started in March 2014 and the date of this post was set accordingly, to have a correct chronological order of all projects.)