Ensure 'modified' field of ArtClassDefinition is always initialized.

This field was not always being initialized due to the use of a
default constructor. This meant we would go down unintended code paths
in some circumstances.

Test: ./test.py --host -j40
Bug: 36602266

Change-Id: Iba3521d4a2fa752a7ba59dcbcbbd30e0c09869ff