shared.css: Canonicalize and simplify background image rules

* Use the same form as in the LESS mixin:
  * Don't embed the PNG version
  * Do embed the SVG version
  * Use the -webkit- form too
* Add a descriptive comment
* Don't use quotes inside url() syntax

Change-Id: I044a04a321f0dbba7687c2ce1681028ed5cf0ea4