Add comment & refactor check for Obsolete class in FindFieldFromCode

Add a comment explaining why we need a check for a class not being an
obsolete class in FindFieldFromCode. It is needed since if we
structurally redefine a class that is in the middle of running its
<clinit> or <init> function the classes final fields will be set from
a function with the obsolete class as its declaring-class.

Also refactor the code a bit to put this logic in ArtField.

Test: None
Change-Id: I9ebb0c89568b9d8e5a60d44c982584b174370eef
3 files changed