Modems may get notified via unsolicited messages (either AT or QMI or some
other thing), that the network time was updated. This setup allows subclasses
implementing the Time interface to setup/cleanup and enable/disable the
unsolicited messages, as well as notify about the network time updates.
Following the same logic as in the original implementation, we try to load the
network timezone information only after being registered, and also with up to N
retries. The async operation in charge of the loading can be cancelled
gracefully, i.e. if the interface is disabled before we get ever registered.