ART: Move Class::Status to ClassStatus

Move the enum to its own file to lower include burden on others.
This is an intentionally small and localized change. Users will
be cleaned up in a follow-up.

Bug: 63467744
Bug: 65318848
Test: m test-art-host
Change-Id: If3bfe6953973e24623e78a2a8dcc69b50117aa3c