[native] Don't prompt SIWE users about keeping login info on logout

Summary:
Context: https://linear.app/comm/issue/ENG-2799/on-native-log-out-dont-prompt-siwe-users-about-keeping-login-info-in

Basically there are no native credentials to be kept or removed, so the existing Alert is irrelevant.

Instead added an "are you sure you want to log out" Alert since immediately logging out (destructive action) without any confirmation was a bit jarring.

Test Plan:
Here's how it looks when a SIWE user taps "Sign out":

Here's how it looks when a "traditional" user taps "Sign out":

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6831