Flag stashing desktop when going home

Put logic to stash desktop apps behind a flag. Turn it off by default.

Bug: 292109910
Flag: persist.wm.debug.desktop_stashing
Test: turn off the flag, open an app on the desktop and go home
      observe that user is taken to home screen and subsequent launches
      are in full screen
      turn on the flag, open app on the desktop and go home
      observe that there is a toast shown for desktop and next app is
      launched to desktop
Change-Id: Ic6f9a3644bb09d4fb877f4694843a7c20969f925