[lib] Don't bump thin thread timestamp for membership robotext

Summary:
This addresses ENG-9558 for membership robotext messages. The implementation is broadly similar to D13829.

Depends on D13918

Test Plan: Tested in combination with the rest of the stack. See video in D13918

Reviewers: tomek

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D13919