Don't create PC/SC permission prompts on Login Screen

[emaxx-google]

Permission prompts are useless on ChromeOS Login Screen because the user (intentionally) doesn't normally see them. Even if they saw them, ChromeOS wipes out all storage every time the Login Screen is loaded, also login-screen extensions are shut down when there's an active user session.

The only reason we create them today is because this allowed the code to be simpler and consistent with the in-session behavior.

[emaxx-google]

One thing to keep in mind: when changing the code to not show the dialogs, we should still not bail out immediately like in the "user canceled the dialog branch". Instead we should "hang" the requests indefinitely, because it's possible that after some time admin policy arrives that force-allows a client extension.