set infinite timeout for urgency notifications

Currently, clients can't set a timeout. All expiration timeouts are hard coded. Did several tries by using the expire_timeout here: https://gitlab.com/lduboeuf/lomiri-notifications/-/blob/ubports/xenial/src/NotificationServer.cpp#L186 but as clients sometimes have a -1 set for timeout, it will have lots of side effects. The idea is to define an infinite timeout for urgency notifications only. That is something stated in the freedesktop spec

fixes #1 (closed)

CI build: https://ci.ubports.com/blue/organizations/jenkins/UBportsCore%2Flomiri-notifications/detail/MR-8/1/pipeline