SystemUI: fix flicker when removing tiles

Instead of clearing all tiles and re-adding them all one by one when
adding or removing tiles, we'll cache the ones already present and
re-use them to avoid having to lay them out from scratch.

Change-Id: I6d083337c08aca65f5ba8073f53c486bff6045a0
Signed-off-by: Roman Birg <roman@cyngn.com>