Remove fullscreen from previewer and charts (issue 2070)

Yet another tease :smile: This change removes the fullscreen mode from both previewer and charts.
While I fully support to keep the setting for the reviewer (for another reason than screen estate, though, it just allows for a more distraction-free review process), I don't see much value in it in the other activities. The previewer always switches into fullscreen, which is a bug identified in [2070](https://code.google.com/p/ankidroid/issues/detail?id=2070), and in the stats fullscreen is the only menu button...