Move content of library 'core' into main project 'app' because the
library was creating problems for no gain (since the code is
not reused anywhere and will most likely never be). The main problem with the
library was that I couldn't access android features because it was a
java library. I could have changed to an android library, but instead I
just migrated the code towards the main project because there was no
point in continuing to maintain the library.