Fixes for mirror-disconnect transition.

Removes the unnecessary transition check for ChangeDeskDisplay
transitions as these prevented Shell-started disconnects from being
handled.

Removes independent handling of desk activations on displays with pending ChangeDeskDisplay transitions on that display, in favor of letting the repository handle activations when that transition is processed instead.

Additionally, dedupes the canCreateDesksChanged check in DesktopRepository.

Bug: 431119389
Test: Manual
Flag: com.android.window.flags.enable_display_disconnect_interaction
Change-Id: I86cc479ca2dbe3648bcee182efd3d865e587ce29