Reorganize variables to enable overriding

This patch enables scss variables to be overriden. It also cleans up unnecessary variables
defined in _variables.scss because of previous wrong overriding.
More details on

Closes-Bug: 1390101
Change-Id: I5078fe46ea6a992a096e2e23118a20cb199b7942
Merge "Reorganize variables to enable overriding"