Add a content notification service to bling.

Bug: b/326254114
Change-Id: I7d4dac4dd0093ab0b414489f5d89b4abb5f407ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5315004


Reviewed-by: Sergio Collazos <sczs@chromium.org>
Commit-Queue: Marke Hallowell <wry@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1279217}