obex: Make initial notifications for Bluetooth incoming transmissions CRITICAL

By making these change, interaction with these notifications will be
more intuitive for the user, as they won't ever expire until being
explicitly dismissed. Also, notifications will show all the information
plus the buttons required to make a decision, right from the start.

From https://developer.gnome.org/notification-spec:

 "Critical notifications should not automatically expire, as they are
  things that the user will most likely want to know about. They should
  only be closed when the user dismisses them, for example, by clicking
  on the notification."

https://bugzilla.gnome.org/show_bug.cgi?id=739426