Add channel to notifications.

Summary:
Oreo requires that each notification has a channel assigned. This patch uses a common channel for all. Individual channels for different notifications (Notifications plugin, pairing notifications, share plugin) could be done, but
since in our case notifications are not a critical part of the UX it does not seem necessary to me. Setting the channel on NotificationCompat.Builder requires version 26 of the support library, the implications of this were
discussed in D8966.

Reviewers: KDE Connect

Subscribers: mtijink, albertvaka, KDE Connect

Tags: KDE Connect

Differential Revision: https://phabricator.kde.org/D9514