Fall back to system DEFAULT_DISPLAY on disconnect if needed.

In some cases, user will not have a main display assigned, which causes
a crash in disconnectDisplay as we end up attempting to perform
disconnect on an invalid display.

Fixes this crash by falling back to DEFAULT_DISPLAY, similar to behavior
in TaskDisplayArea#getReparentToTaskDisplayArea

Bug: 433962634
Flag: com.android.window.flags.enable_display_disconnect_interaction
Test: Manual
Change-Id: I7de45beaa484d818acbc0d76a8636d1c3550dbaa