Add dalvik.system.ClassExt class

This class is allocated by the runtime lazily and is used to hold data
for java.lang.Class that is often not needed. This allows us to add
useful things to keep around in Class without regressing memory use.
Initially we are placing the saved verifyError into this class. This
type is expected to grow as time goes on.

Test: m test-art-host

Change-Id: I9556734102d7e7a35a81e8af8b133cba2e75d16c