[native][web] Go to PRIVATE chat when the user selects themselves in chat composer

Summary:
We'll handle this just like when the user selects a non-friend: we'll just open the chat directly.

We we want to treat this case similarly to when the viewer selects a non-friend: instead of letting them include themselves in the list, we will just open directly to the PRIVATE if the user select themselves at the start (this diff), and not list them as an option if they select somebody else at the start (D13543).

Depends on D13544

Test Plan: On both native and web, confirm that when the viewer selects themselves, it opens up the pending thread directly

Reviewers: tomek, kamil, inka

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D13545