[applets/digital clock] Restore scroll-to-change-displayed-timezone

After the rest of the UI changes made to the applet and the time zone
config page, I don't think users will be confused by the differences
between the timezone displayed in the clock, and the systemwide local
timezone anymore. Therefore, it should be safe enough to bring back the
wheely time feature.

BUG: 431977


(cherry picked from commit a699acd3)