Add a new database library for Index V2 and diffs
This MR only adds the database
library (with tests), but does not yet use it. This will happen in the next MR.
API docs are available here: https://grote.gitlab.io/fdroidclient/libs/database/
Also, the download
and index
library are being moved into a new sub-folder for the libraries where also a new sharedTest
module was added to be able to properly share test classes and data across libraries. The old symlink hack isn't allowed in latest Android Studio anymore.