Pressing back button, we don't choose any provider

Once in the provider detail fragment, if the user presses the back
button, provider.json is removed from sharedpreferences (so that
Dashboard does not start as if the user had selected the canceled
provider) and ConfigurationWizard gets the focus.
Back button from ConfigurationWizard quits.

If the user decides not to choose any provider, the app quits. I'll
create another branch to handle this exit properly, warning the user
with an AlertDialog as suggested in #3023.
ProgressDialog works ok when discarding providers.

If the user selects a provider, gets into provider detail, discards it,
further trials of selecting a provider trigger the same ProgressDialogs
than the initial trials.