[audio_service] Change DeviceNotifier from flat_map to RemoteSet

The notifier previously managed the device change observers in
a `flat_map`. This CL removes some of the management steps by
migrating it to use `mojo::RemoteSet` instead.

Change-Id: I8cf57e9aa54c5c6f066dc87fb2a905124098487b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4675106
Reviewed-by: Olga Sharonova <olka@chromium.org>
Commit-Queue: Bryant Chandler <bryantchandler@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1171214}