Pin desktop tile to the right in overview

When desktop windowing prototype is enabled, pin desktop tile to the
right of the focused tile.
Also making desktop tile persistent so it will always show up,
regardless whether there are tasks added to the desktop or not.
Fixing some nullpointer issues that stem from that.

Bug: 261234155
Test: no recent tasks, go to overview, shows empty view
Test: only desktop tasks, go to overview from home, desktop tile focused
Test: only desktop tasks, desktop open, go to overview, desktop tile
  focused
Test: 1 fullscreen task and some desktop tasks, go to overview from home
  screen, fullscreen tile focused
Test: fullscreen task open and no desktop tasks, go to overview,
  fullscreen task focused and empty desktop tile on the right
Test: fullscreen task open and some desktop tasks in background, go to
  overview, fullscreen tile focused, desktop tile on the right
Test: some desktop tasks focused, some fullscreen tasks in background,
  go to overview, desktop tile focused, fullscreen tasks on the left

Change-Id: Ib28185b40f639d36b82b6d0e3c61026ec84e245c