Turn onboarding off by default
Turn onboarding off by default

Fixes test failures in browser/extensions/onboarding/test/browser/browser_onboarding_accessibility.js
AdamHillier/activity-stream,sarracini/activity-stream,mozilla/activity-stream,Mardak/activity-stream,mozilla/activity-stream,mozilla/activity-streams,mozilla/activity-stream,AdamHillier/activity-stream,AdamHillier/activity-stream,sarracini/activity-stream,Mardak/activity-stream,AdamHillier/activity-stream,mozilla/activity-streams,mozilla/activity-stream,rlr/activity-streams,sarracini/activity-stream,sarracini/activity-stream,rlr/activity-streams,rlr/activity-streams,rlr/activity-streams,Mardak/activity-stream,Mardak/activity-stream