require __MockClass of 'abstract final' to be 'abstract final'

Summary: ... so that the invariant that "there are no objects for
 which $obj instanceof AbstractFinalClass" is maintained.

Reviewed By: @paroski

Differential Revision: D1711272

Signature: t1:1711272:1417483818:6eb3486e5036661b2b19174db3139b3cefe6bbe2