[native] Show your PRIVATE thread at the top of search results

Summary: Follow-up to D1016. Seems like we should prioritize PRIVATE instead of keeping it at the bottom with the rest of the non-PERSONAL threads.

Test Plan: Make sure PRIVATE is at the top of search results on native

Reviewers: palys-swm, KatPo

Reviewed By: palys-swm, KatPo

Subscribers: Adrian, atul

Differential Revision: https://phabricator.ashoat.com/D1017