Show ArtMethods in imgdiag

Since ArtMethods were moved out of mirror:: classes imgdiag does not
show information about them.  Diff ArtMethods to facilitate finding
dirty memory there.

Bug: 38173645
Test: imgdiag --boot-image=/system/framework/boot.art --image-diff-pid=`pid system_server`
Change-Id: Icd86a9ef14d5177a297026c22c81c080f5c85fc1