Convert steps to array of ints and floats, not strings.

Fix for [Issue 1777](https://code.google.com/p/ankidroid/issues/detail?id=1777)
The StepsPreference currently returns a JSONArray of strings, and we end up storing them that way in the collection. While this works on AnkiDroid, it breaks the desktop client during review since it's expecting numerical types. This will ensure we return a JSONArray with numerical types.