Desktop: check wallpaper to know if last window is closing

Before this CL, to know whether Desktop Mode is closing, we would check
whether the closing task is the last active task in our desktop
repository.  However, in cases where we close the last task while the
second to last task is still closing there will still be two active
tasks in our desktop repository.

In this CL we instead determine whether desktop mode is closing by
checking whether the wallpaper activity is closing.

Bug: 379852092
Test: ensure closing last task while 2nd last still closing works
Flag: com.android.window.flags.enable_desktop_windowing_exit_transitions
Change-Id: I7e999e6baf4e809409f54d7093f0b32609866f38

Change-Id: Iab045e98e0e5f88925dfc4f0dd7166494910269b