EditableTable: decide row expansion with a predicate

The expanded-rows record was derived state: rows plus what the user
toggled. A predicate matches the neighbouring getRowKey/getRowVariant
hooks and lets a caller supply its own default per row.

Change-Id: I8345a0a7b36ac82da2264152580e0f52c42be0fa
JIRA-Ref: CMK-38111