Center placeholder texts in system tray popups

Summary:
We use this centered style in many other places and are moving towards using it
everywhere. This patch applies the same style to the System Tray popups for
notifications and devices, which were still using the old style.

Test Plan:
Notifications, before:

Notifications, after:

Device Notifier: before:

Device Notifier, after:

Reviewers: VDG, Plasma, ndavis

Reviewed By: VDG, ndavis

Subscribers: davidedmundson, ndavis, trmdi, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D27219