Air/Oxygen desktoptheme: fix hand offsets with hint-*-rotation-center-offset

Summary:
The hands of the clocks still relied on the KDE4 times rule that the
position in y-axis relativ to the clock center would define the relative
rotation point for the hands.

With the default in Plasma 5 being (width/2, width/2) from topleft of the
hand shape, using hints of upcoming Plasma 5.16 allows to override that
again to match the original definition, without the need to enlarge
the pixmaps of the hand shapes.

Reviewers: Plasma, mart, ngraham

Reviewed By: ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D20496