ART: Simple structural class check

Adds a simple check to class-loading when the embedded dex file in
an oat file and the dex file on the class path where we found the
class do not match.

We require that the number of methods and fields do not change, as
that will almost certainly mean that quickened and other compiled
offsets are wrong now. This is a reasonably lightweight change, but
we should investigate a full comparison including name and type of
members.

Bug: 17937814
Bug: 18708951
Change-Id: Icb9638bebd369ab23822817f4a97c8dd8625fea5