Fix bug where icons in hotseat were clipped when display size is Small.

Simplify calculations to ensure the hotseat sizes are always accurate.

Bug: 186556930
Test: changed display setting from small to largest on different grid
      sizes (5x5, 4x4, 4x5, 3x3, 2x2)

Change-Id: I97580c50a9d821577eb63335950d5695d9bf035d