Only perform mExistenceChanged check on DisplayContent changes.

Since non-display changes can have mExistenceChanged set to true, checking it in ChangeInfo#hasChanged results in unexpected transition behavior and test failures. Since we're only concerned about the existence changes of display, move the check into applyDisplayChangeIfNeeded.

Bug: 391652399
Test: Manual, atest
Flag: com.android.window.flags.enable_display_disconnect_interaction

Change-Id: Ic65e01a2b1a4ba7760fc50cbefb618ba3e60d04e