Move on from Android's content provider API to plain ORMLite API.

Actually, it's really a pain to handle both content provider API and
ORMLite (or other mapping librairies) at the same time. It requires lots
of boilerplate code to transcode between plain POJO and Android's
ContentValues and Cursor.

Content Provider implementation will stay here to implement sync adapter
but the whole UI layer will directly use ORMLite API instead of
ContentProvider API.