stats: removed old statistics code, changed stat menu and title

First: I had to make a manual merge because StudyOptionsFragment was changed upstream. I hope everything went well.

In this PR I have removed achartengine and the old ChartBuilder class. If achartengine has to be used in the future, a newer version should be used probably. 

I'd prefer staying with my version of course. To make that better maintainable I intend to write a documentation on how to add or alter a graph. That would cover use of my library. But first comes the coding, especially bug hunting when more users get to use the new stuff. 

I edited all classes that still had references to the old statistic code including DeckTask and removed most of it. At some points I wasn't sure so I added my statistic code instead of a full removal (in StudyOptions somewhere).

I changed the deck and time selector buttons in statistics to simple submenus. Deck selection is now only done in the DeckPicker so only options are current deck or whole collections (like in desktop). That works better with menus since I can't use an Adapter for the deck names to fill a menu.

Also there is a subtitle now with the deck name. For conformity and because @timrae asked me to I also changed the title in StudyOptionsFragment to use the Activity name (as a string resource that I uncommented) as title and the deck name in a short variant as subtitle. (I can use the app name for the title instead, I'm not sure what is best).

Oh yes I added some transparency to the statistics to make the look more like the desktop variant. I think the breakdown charts benefit from that.