[native] Create more generic other member id finder

Test Plan: I verified that it works correctly by blocking a user in a personal thread and checking that typing a message is blocked. Also opening pending thread and creating a personal thread by sending friend request and sending a message works as expected.

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: KatPo, zrebcu411, Adrian

Differential Revision: https://phabricator.ashoat.com/D487