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ClassifyActivity: Use launchMode=singleTop.

So the Up button (from Help/Examples, or List, for instance) is more
like the back button. Otherwise, Up navigation (correctly) ignores
the back stack and just creates a new instance. But our
ClassifyActivity has lots of state that we don't want to lose.
Alternatively, we could have used FLAG_ACTIVITY_CLEAR_TOP when
navigating up, but this seems simpler.

I do wonder exactly how this is dealt with in standard apps,
such as when writing an email, pressing a button to launch a photo
chooser activity and then pressing Up to come back, without losing
the contents of the email. However, I cannot find a real-world
example like that that actually has an Up button in the
helper activity.