Fix FAS config to persist across login calls

Summary: The previous implementation of SetFastAppSwitchEnabled (D91556255) created a FBSDKLoginConfiguration to test the appSwitch parameter but never stored the value or wired it into the actual login paths. The config was not wired frough when the function returned, so calling SetFastAppSwitchEnabled(true) did not dynamically change login behavior correctly.

This fix:

• Adds a static BOOL to persist the appSwitch preference across calls
• Threads the stored value into both login config paths in loginWithTrackingPreference (with and without nonce)
• Adds Enable/Disable FAS buttons to the MainMenu example scene for easier testing

Reviewed By: Fobertree

Differential Revision: D99152986

fbshipit-source-id: 01b82f6d2897bf7e3149ef927ee0891f287b933d