[lib] Include threadType in getPendingThreadID

Summary:
I noticed an issue where we would show a PERSONAL createPendingThreadItem in the search results for users for whom there was no existing PERSONAL thread, but when the thread was pressed it would turn into an existing non-PERSONAL thread.

This is definitely an edge case (you should usually have a PERSONAL thread before a non-PERSONAL thread with a user), but it's possible if somebody adds both of you to a CHAT_SECRET thread and then leaves.

I think the right solution is that we should include threadType in the pending thread ID, so that a pending thread ID of a particular type can only be replaced by a realized thread of the same type.

Test Plan: Make sure that pressing a PERSONAL createPendingThreadItem in the search results doesn't redirect to a realized non-PERSONAL thread

Reviewers: palys-swm

Subscribers: KatPo, Adrian, atul

Differential Revision: https://phabricator.ashoat.com/D1094