move default keystore location to `keystore.jks`, i.e. in the fdroid repo

move default keystore location to `keystore.jks`, i.e. in the fdroid repo

This makes `fdroid init` create a complete, self-contained repo in a single
folder.  That makes it easily transferable, backupable, etc.  It also means
that `fdroid update` can add a keystore to an existing unsigned repo
without having to worry about finding the right folder or overwriting any
other existing files.