Default fast app switch ON for LoginWithTrackingPreference

Summary: The Unity iOS bridge initialized _fastAppSwitchEnabled = NO and always passed an explicit appSwitch into FBSDKLoginConfiguration, forcing .disabled unless the app opted in. This suppressed the native iOS SDK’s opt-out default (appSwitch = .enabled) and diverged from the legacy logInWithPermissions: path, which omits appSwitch, inherits the default, and is already fast-app-switch (FAS) on. This diff removes the divergence by defaulting _fastAppSwitchEnabled to YES.

Scope: the flag gates loginWithTrackingPreference for both tracking modes (classic enabled and limited). The legacy LogInWithReadPermissions path already inherits the default, so it needs no change. RefreshLimitedLogin is unaffected – it is governed by RefreshFallbackPolicy. Android’s SetFastAppSwitchEnabled is a no-op and is left untouched. The bundled internal/samples/TrashDash copy of FBUnityInterface.mm still initializes to NO; it is a vendored sample snapshot refreshed separately, and is deliberately out of scope.

Behavior change: eligible logins now app-switch to the Facebook app when the FB app is installed and the FAS GateKeeper is enabled, instead of using the web flow. Apps can still opt out via FB.Mobile.SetFastAppSwitchEnabled(false).

Differential Revision: D119191567

fbshipit-source-id: 913f8d5f26c273117ff94eb0152a03e02356cf0c