fixed crash in addVacantCell

No longer precalculating vacant cells

The previous way of finding empty cells for 
widgets/icons etc. precalculated all the possible
empty spaces. Now that we have an 8x7 grid in
tablet, there are too many possible empty spaces.

Change-Id: Ib39113fdf755935bfad257843e1618c680ed9e72
3 files changed