MD Settings: refactor some main page properties, fix Advanced toggle

Fixes the Advanced toggle when navigating directly to Advanced.

Make currentRouteChanged_ less confusing. Wrap non-About stuff in a dom-if.

Removes a bunch of redundant properties.

Part of https://codereview.chromium.org/2106013002/

BUG=630010,589681
R=dbeam@chromium.org
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2160713002
Cr-Commit-Position: refs/heads/master@{#407236}
2 files changed