Disconnect transition test fixes.

To ensure Transition#applyDisplayChangeIfNeeded picks up the display change, flag the DisplayContent as changing existence.

Additionally, create a transition flag for display-level transitions to prevent them from becoming no-op in TransitionController#queueTransition.

Third, add a validator during display removal to ensure the DisplayContent is removed even if the transition fails to complete.

Bug: 391652399
Test: atest, cts
Flag: com.android.window.flags.enable_display_disconnect_interaction
Change-Id: I31abb1d745b0e91abb6017a2068bbb77b63d6ebb