Define BasePayloadAnalyzer::SERVICE_NAME

Summary:
As the constant is used in BasePayloadAnalyzer methods, we define it
here too, with a UnknownService value to help debug.

Test Plan: phan | grep SERVICE_NAME

Reviewers: dereckson

Reviewed By: dereckson

Subscribers: alken-orin

Differential Revision: https://devcentral.nasqueron.org/D776