Annotate $classOverhead arrays.

ART adds a fake byte[] $classOverhead static field to classes to show
the overheads associated with the class. This change causes ahat to
annotate those $classOverhead arrays with the class that they are
overheads for.

Test: m ahat-test, with new test added.
Test: Open an Android heap dump and visually inspect:
      http://localhost:7100/objects?id=0&heap=app&class=byte[]
Bug: 64832332

Change-Id: Ief6ed7ce6c8c1196bc644df36f03c8e5158bf658