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This CL add a callback to sendStandBy() and sendWakeUp() that is run
once requests have been been made on D-Bus to put displays to sleep
or wake them up.

Additionally, an API test is added ensuring that calls to the functions
propagate to the D-Bus client.

BUG=b:78171027

Change-Id: I7c529ce328bcd8ce67d96d357f71dfbc0da229cf
Reviewed-on: https://chromium-review.googlesource.com/1053780Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Toni Barzic <tbarzic@chromium.org>
Commit-Queue: Felix Ekblom <felixe@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561451}