[Nearby] Add observation interface to service

This CL adds an observer interface to allow clients of the service to
be notified when high visibility state changes as well query the current
state. Previously, high-visibility was not part of the api. It was an
implied state when a foreground receive surface was registered and
advertising was is started successfully with a device name in the
end-point info. This change makes it possible to query and monitor high
visibility so UI surfaces can be in sync with the service state.

Additional methods will be added to the observer interface in follow up
CLs (i.e. Shutdown).

Change-Id: I40b718fb9fb1ab185ef45a4ff0085d4c59c8bad5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2401851


Commit-Queue: James Vecore <vecore@google.com>
Reviewed-by: Ryan Hansberry <hansberry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805888}