Summary: As reported in https://fb.workplace.com/groups/748354712423318/permalink/773382183253904/, special case no devices being present and suggest to launch an emulator. Also extends `useStore()` hook to accept no arguments, in which case it works as Redux's own `useStore` hook, except that it is strongly typed. Reviewed By: passy Differential Revision: D25803497 fbshipit-source-id: d448ac41e0ba7b713ee883caeb27736a2901975c