Stash desktop apps on home gesture or taskbar home button press

When we detect the home gesture or tap on home button in taskbar,
check if desktop apps are visible. If they are, signal WMShell
to stash the desktop apps.

Flag: persist.wm.debug.desktop_mode_2
Bug: 261234402
Test: have gesture nav enabled, put an app on desktop and swipe up to
  home, verify toast is shown, and next app launches to desktop
Test: have 3-button nav enabled, put an app on desktop and press home,
  verify toast is shown, and next app launches to desktop
Change-Id: I824e640bfafa20cb3451cbd8f035fe9dca18094f