Fix widget drop preview outline on 4x5 grids

=> We were not account for mBorderSpacing in the outline calculation
=> This was fine in most cases as this value is 0, but 4x5 appears to be the exception

Bug: 194227752

Test: manual, see video in bug.
Change-Id: Ib590bc142a0f65e75f92ee057f8b4f4b0f81521b