Include border spacing to align tasbar icons with hotseat.

The original calculation was including spacing in the the
hotseatCellSize, but that causes us to incorrectly compute the
hotseatIconCenter. If we take advantage of
DeviceProfile.calculateCellWidth, we get the width without the spacing.
Then we can add spacing for every icon to the left of the current one
when figuring out the center.

Test: Manual on tablet using device types tablet and multi display.
Fix: 210123477
Change-Id: Ie182718ad3a229ffa8bae031f3ac7f73f8539f49