Support alt+tab for desktop tasks

Updates keyboard quickswitch logic handle quickswitching while on
desktop. When on desktop, alt+tab moves only between desktop tasks.
Fullscreen tasks are shown in the overflow tile and can be accessed by
going to overview.

TODO:
- when not in desktop and using quickswitch, show the desktop tile as a
  combined tile in the row, similar to what is shown in overview

Flag: persist.wm.debug.desktop_mode_2
Bug: 280468885
Test: open some apps on desktop and have some fullscreen apps opened in
  the background, when on desktop, observe that alt+tab only switches
  between desktop tasks
Test: same setup, but switch to a fullscreen app, observe that alt+tab
  only switches between fullscreen tasks and desktop is accessible from
  overview

Change-Id: Ib19f2eaa24363bbd0669c8c8c3d99ed9d9118a17