ContextHubService: Flag test mode duplication

This CL adds the flag for handling reliable
message duplication guarding the test mode
behavior for message duplication so the test
mode event only happens if the service supports
handling it.

Bug: 331795143
Change-Id: I223b05d63485b5ab1be9c90c39e8bdb6b889933b
Flag: context_hub.reliable_message_duplicate_detection_service
Test: Presubmits