mobile notifications: update observers

Stop subscribing to all network property updates.

Subscribe to default network changes to show a notification when
cellular becomes the default network.

Subscribe to session updates to show the notification if cellular
is the default network after a session change.

We avoid showing the notification if there are other network
connections pending. Subscribe to network connections changes to make
sure we don't accidentally miss legitimate updates.

Bug: 910767
Change-Id: I189b16cb5fc19428c13848e7eb193952ca6655ac
Reviewed-on: https://chromium-review.googlesource.com/c/1399428
Commit-Queue: Tony De Luna <tonydeluna@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622638}
6 files changed