[WE] Enable Welcome Experience feature flag by default

- This change also updates the notification_controller to use a generic
  preference for device notifications to avoid duplicate notifications
  for certain devices, i.e., keyboard-mouse combo devices.

Bug: b/329686601
Change-Id: I4dc31ad2abc1df1c5cbdeaee36fcce90c58eb679
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5793742
Reviewed-by: David Padlipsky <dpad@google.com>
Auto-Submit: Michael Checo <michaelcheco@google.com>
Commit-Queue: David Padlipsky <dpad@google.com>
Cr-Commit-Position: refs/heads/main@{#1343119}