[MD settings] move footer class into dialog style-module

This CL doesn't change the setting UI.  It pulls (and adds)
some dialog settings into the dialog style-module.  This CL
also changes some of the color settings to use polymer color
variables.

BUG=589630

Review URL: https://codereview.chromium.org/1774283002

Cr-Commit-Position: refs/heads/master@{#380172}
4 files changed