Fix ReorderableListView's use of child keys (#41334) (#41338)

ReorderableListView was constructing a GlobalObjectKey using
the child key as the value. This only had the intended behavior
if the child key was identical across build method invocations.

The new strategy is to scope the child key's value to the
State object's identity, allowing child keys to have value
compare semantics while disambiguating among different list view
instances.