shill: Use SuspendDone signal instead of PowerStateChanged.

PowerStateChanged will be removed from powerd soon; clients
should use SuspendImminent and SuspendDone instead.

Also simplify the interface exposed by PowerManager for
adding and removing suspend delays.

BUG=chromium:359619
TEST=updated tests

Change-Id: I7f96774570170591c2e5944245c368e165aaf972
Reviewed-on: https://chromium-review.googlesource.com/195690
Reviewed-by: Daniel Erat <derat@chromium.org>
Tested-by: Daniel Erat <derat@chromium.org>
Commit-Queue: Daniel Erat <derat@chromium.org>