Use <img> instead of <iron-icon> in Assistant optin

Also:
- update logic for Assistant settings.
- update timeout for loading the flow.

Bug: 866141, b/111704810, b/111796422
Test: Manual Test
Change-Id: Ibb3ba413992ee92b2e334dddeaf7d714d500ffcb
Reviewed-on: https://chromium-review.googlesource.com/1148768
Commit-Queue: Yue Li <updowndota@chromium.org>
Reviewed-by: Scott Chen <scottchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577792}
15 files changed