Update hotseat sizing for 4x5 grids.

If the qsb bottom margin is smaller than the bottom insets,
we'll fall back to centering the qsb in the available freespace.

Bug: 194435265
Test: 4x5 grid
      multiple devices,
      with: gesture nav enabled
            3 button nav enabled

Change-Id: I9fd4f0da3f2bc9f0b38a621ad1192533260379e3