Add bottom padding to hotseat in multi-window mode.

In multi-window mode, the navigation bar is black which makes the
hotseat's top padding -- and lack of symmetric botom padding --
super apparent. We add bottom padding to the hotseat so that the
hotseat icons remain visually centered.

Bug: 32176631
Change-Id: I3eebf5ab98898c8ee092bda6ec48c6afdcd353b9