Add border spacing and fixed cell height to grid.

- Border spacing is the spacing between the cells.
- Workspace cell height is now fixed, and we allocate
  all the "extra" space to three different variable height
  areas.

* Built behind ENABLE_FOUR_COLUMNS flag because it hinders the
default grid.

Bug: 175329686
Test: - set border spacing to 0 and confirm matches prior layout
      - test drag and drop still worked
      - test reordering
      - test widgets
      - test folders
      - test multiwindow

Change-Id: Ic6f3dff577d28ff214bda4b0a787ec7fd08c108b