Don't process external classes
Don't process external classes

Summary: Do not set keep bits on external classes. This caused a crash for the target automation_fbandroid_redexunstable_fallback_release

Reviewed By: dariorussi

Differential Revision: D4137651

fbshipit-source-id: b0ffe4232fc368a1d0b9b99a2d5f8c2e6eae0d21

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