Return the existing DisplayAreas when registering an organizer

A list of existing DisplayAreas of the given feature are returned when
registering the organizer, and the organizer will not receive
onDisplayAreaAppeared() for these DisplayAreas.

Fix: 162028888
Test: atest WmTests:DisplayAreaOrganizerTest
Test: atest OneHandedDisplayAreaOrganizerTest
Change-Id: I3f4c5921a31c06e5b6c0040397df15cdee6ebcdd