Fix //conditions:default excludes computation for LabelListAttribute.

Previously, excludes computation for the default condition in label list
selects was clobbering pre-existing values. This CL fixes it by
performing a union of the new values with existing ones instead.

Test: properties_test.go
Bug: 198556411
Change-Id: Id11f4fb14e359201304afde0d8ba856851d41395