launcher: use scalable grid in 4x4

Many changes are required to make scalable displays work correctly on
foldables. This first one is making sure that the correct number of
columns is used when calculating the used width for scalable grid. The
spaces around the workspace are not final yet.

Bug: 191879424
Test: manual
Change-Id: Idc41ed004580f1f86d8f9595d005abc72301b1e3