[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
[1/3] base: allow disabling non intrusive incall ui

Change-Id: Ibe9c65f1fe024a859d98d96fd86199607a1060de

Patchset: 1
https://gerrit.omnirom.org/#/c/3047/