android: Show a confirmation dialog before importing certificates

android: Show a confirmation dialog before importing certificates

Since the import activity can be triggered by any other app on the
system we shouldn't just import every certificate we get.

Also, in some situations (e.g. if no passphrase has been set yet for the
system-wide certificate store) we are the only application that can open
certificate files.  So if a user clicked on a certificate file she would
just get a confirmation Toast about a successful import, with no indication
whatsoever where the certificate was actually imported.  The new dialog
shows the app icon to indicate that strongSwan is involved.