Check that classes are resolved in GetMethodsVisitor

VisitClasses doesn't internally check that classes are resolved. This
caused GetClassesAndMethodsVisitor to occasionally see ArtMethods
that didn't have their declaring class filled in.

The fix is to check that the class is actually resolved.

Test: test-art-host
Bug: 62585606
Merged-In: I3ed647ae15900c96d2180eb5c799f45393794dda
Change-Id: Id09e9e27cf09a921fbce4c917b8b6ef2daeaabaa