Have DesktopWallpaperActivity finish on disconnect.

DesktopWallpaperActivity should never be able to change displays, and in some situations, the removal in DesktopTasksController#addOnDisplayDisconnectChanges is unsuccessful, leading to default disconnect logic moving the activity to the default display.

Fix this by having the activity finish when its display is removed or when attempting to move it to another display.

Bug: 434198011
Bug: 432494930
Test: Manual, disconnect display on phone, no wallpaper activity remaining
Flag: com.android.window.flags.enable_display_disconnect_interaction
Change-Id: Ifb26c32ee7135b1def4f92d6d5a6a74fa4f2003f