shill: PropertyChangeNotifier: Initial commit

Add a PropertyObserver, whose task is to track changes
to the returned value of an accessor.  Further, create
a ServicePropertyChangeNotifier, whose task is to monitor
a collection of PropertyObservers for a Service and call
the appropriate Emit*Property method for those properties
which have changed.

BUG=chromium:379948
TEST=Unit tests
Change-Id: I450b74b388f8ecb44010f277cc77149412b24950
Reviewed-on: https://chromium-review.googlesource.com/202441
Reviewed-by: mukesh agrawal <quiche@chromium.org>
Commit-Queue: Paul Stewart <pstew@chromium.org>
Tested-by: Paul Stewart <pstew@chromium.org>