[auth-swift] Fix two flow porting errors in AuthURLPresenter.swift

[paulb777]

While doing exploratory testing investigating the GitHub provider workflow for #11759, I discovered that dismissing the sign in window didn't properly reset to try again and then once fixing that, the dismiss didn't happen in the main thread.

Fixed by implementing the missed SFSafariViewControllerDelegate delegate method - safariViewControllerDidFinish and fixing the async queue for the dismiss method.

[google-oss-bot]

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