Currently the view to show after pressing the back button is defined
by individual InnerView classes. For example, the JoinWifiView always
says its previous step is the SelectAppsView. This works for the most
part, but doesn't work for:
* The first StartSwapView class (instead handled by a special case in
the Activity).
* WifiQrView which could either be going back to the NfcView or the
JoinWifiView, depending on a few cases, which the WifiQrView probably
shouldn't care about.
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