Change StateChangeQueue::NotifyPropertiesUpdated to take const ref

Instead of using std::unique_ptr<>. This simplifies code around the
usage patterns and tests, and causes only one extra copy of the
dictionary in one case.

Change-Id: Ic0d3158116a7be5823078b08f1bf58e1c88c88b7
Reviewed-on: https://weave-review.googlesource.com/1769
Reviewed-by: Vitaly Buka <vitalybuka@google.com>