ART: Add class instance fields in hprof dumps

Emit synthetic static fields representing class instance fields
in the class dump tag. Naming starts with "$class$."

Refactor code slightly to share writing code with regular static
fields.

Bug: 38167721
Test: art/test/testrunner/testrunner.py -b --host -t 130
Test: manual (use ahat to inspect an hprof dump)
Change-Id: I00af60f7b92b737dc69e29a7ef0532f65516eaf7