[1/2] 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: Ie3321e9a3e0637d6ba8094dd46aee36c41354713
SystemUI: Fix notification header disappearing on tablets

For some reason, the notification panel header has weird dimensions
when it has anything else than its original ColorDrawable as
background, and only on tablets.

We workaround this by not changing the drawable until requested.

Change-Id: Id76a5dcacf59c35b2e49681d6eeb4e828f7cbc58