FileHandlerActivity: don't finish if showError() will be called

If everything works, we want FileHandlerActivity to disappear/finish as
fast as possible.  But if it needs to show an error, then the dialog's
OK button should trigger the finish().

Now that the error dialog shows, we don't need the extraneous Toast.
FileHandlerActivity: make sure to close Cursor after query
handle content:// URIs in Intents that don't set the MIME type

OI File Manager sends files in SEND Intents with an odd content:// URI and
does not seem to set the MIME type, so these tricks are necessary to handle
SEND Intents from OI File Manager.

refs #1646
add SignFileActivity for signing incoming files with default key

There wasn't any way to sign files yet, this is based on the approach of
EncryptFileActivity and just provides a very simple interface to the
signing procedure.
prompt to sign/encrypt when receiving non-PGP file

A file:// URI was already routed to encryptFile(), this adds a rough
prompt to choose Sign or Encrypt for any non-PGP file received via file://
or content://

refs #1646
use IOUtils.copy() for simpler, more reliable stream copying

It is really silly to have to implement this kind of thing again and again,
luckily Apache Commons IOUtils has done it for us.
REBASE prompt to sign/encrypt when receiving non-PGP file
REBASE add cmd line "list keys" for debugging