fix handling of volatile proguard keyword
fix handling of volatile proguard keyword

Summary: It seems our proguard parser wasn't working on `volatile` fields.

Reviewed By: minjang

Differential Revision: D16060613

fbshipit-source-id: 10eb677e94ae33ac671c5b66e8bb0d697d3e90a3

facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex