Add jdduke@ to services/proguard.flags OWNERS

This file is only used in optional optimized builds, and will likely
undergo significant churn in the coming weeks. Add jdduke@ as an
OWNER for this file only to help expedite review.

Bug: 210510433
Change-Id: I46d3dfa2d3fc56acd849aa93e3361ac3af5c1341
Test: N/A