Show a persistent toast when selecting desktop apps on home screen

When desktop stashed state changes, show a persistent popup on home
screen to select an app that will be launched on desktop.

Flag: persist.wm.debug.desktop_mode_2
Bug: 261234402
Test: launch an app on desktop, swipe home, observe the popup, launch an
  app, observe app is launched on desktop and popup is hidden
Test: stash desktop apps by going home while on desktop, press on the
  "exit desktop" button in the popup, observe popup is hidden, launch an
  app, observe it is launched in fullscreen

Change-Id: I66fe0ab977fa7b2059f149d7d0ab0cf92192c967