Ensure new encryption key is generated every time iOS/Android application creates database for the first time

Summary: This differential ensures that every time application is about to create new database new encryption key will be used even if there is already one in SecureStore. It is implemented in CommonCpp but it targets iOS. On iOS expo-secure-store is implemented on top of Key chain services which survive application deletion and there is no way we can disable this (see discussion https://linear.app/comm/issue/ENG-552/remove-data-from-secure-store-when-uninstalling-application). It was agreed then that upon app reinstalation we will generate new encryption key (see discussion https://linear.app/comm/issue/ENG-552/remove-data-from-secure-store-when-uninstalling-application)

Test Plan: place two logging lines - one before file_exists(databasePath) check and one after. Ensure that every application reinstallation produces two different encryption keys in those two lines.

Reviewers: tomek, atul, ashoat, jon

Reviewed By: tomek, ashoat

Subscribers: ashoat, adrian, abosh

Differential Revision: https://phab.comm.dev/D4664