ObjectChooser: preserve filter type

When the ObjectChooser is called with the filter_type=FILTER_TYPE_MIME_BY_ACTIVITY
the filter_type was replaced by FILTER_TYPE_ACTIVITY.
This patch preserve the value used, if not filter type was selected,
use FILTER_TYPE_ACTIVITY.

Signd-off-by: Gonzalo Odiard <godiard@sugarlabs.org>