Desks: Let DesktopWallpaperActivity be focusable with empty desk

When there are no desktop windows on top of the
DesktopWallpaperActivities, input events (such as keyboard shortcuts)
could result in ANRs because the focused app (the desktop wallpaper) did
not have any windows to send the input to. See go/anr-book.

With this change, the activity is allowed to be focusable only when it
is the top-resumed activity in the system, which is equivalent to it
being visible with an empty desk.

Flag: com.android.window.flags.enable_multiple_desktops_backend
Fix: 402270860
Test: with 2 desks (1 non empty, 1 empty), switch back and forth with
keyboard shortcuts a few times and verify there is no ANR.

Change-Id: Idc7ac2c374bee72bb54bd7b3a5daca915262af25