[2/2] SystemUI: Add time-context headers to the notification header

This commit adds image headers to the notification panel header,
based on the date/time. There are pictures for morning, afternoon
and night, as well as special events like Christmas and new
year's eve.
More will be coming, with eventually a way to use an external
(as in, Internet) provider for those pictures.

Change-Id: I7e59fc8e23a9fd84f1196c63d066710671c1aada
Merge "[2/2] SystemUI: Add time-context headers to the notification header" into pac-4.4