Make getActiveNotifications async

This CL makes getActiveNatifications() call async for improving notification
performance as it will block  browser thread.

Bug: b:242038130, b:242028843
Change-Id: I6e5c8ac0a21b5e57ea753ea50c8277cc9d8d282a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5405074


Reviewed-by: Min Qin <qinmin@chromium.org>
Commit-Queue: Min Qin <qinmin@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1280367}