OOBE - Add suffix to assistant custom element

The custom element used for the Assistant screen now has
a proper "-element" suffix to discern the dom-module from
the rest of its uses throughout the code. This is a necessary
change for using the HTMLImports polyfill in order to avoid
clashes in the names.

Bug: 1111387
Change-Id: Ib4ec56b74e2c9f035dc1a3166be8fd9b80a24b5f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2544543
Commit-Queue: Renato Silva <rrsilva@google.com>
Commit-Queue: Denis Kuznetsov [CET] <antrim@chromium.org>
Reviewed-by: Denis Kuznetsov [CET] <antrim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828280}