Prompt for login by default
Prompt for login by default

Summary:
This aligns behaviour with non-sandy behaviour of login: Prompt for login on startup if the user isn't yet signed in.

Logic is based on https://www.internalfb.com/intern/diffusion/FBS/browse/master/xplat/sonar/desktop/app/src/chrome/UserAccount.tsx?commit=1fc905fdd1cd&lines=94

Reviewed By: jknoxville

Differential Revision: D24922397

fbshipit-source-id: b4bab9c109a1054c0e260599d28320699c47dfda

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