Revert "Revert "Propagate new link-status-change message to any NetworkManagementService""

This reverts commit 1a7e6719.

Bring back the changes from Stan Chesnutt regarding link-status-change
notifications.  The comment from the original patch was:

Propagate new link-status-change message to any NetworkManagementService
observers.  Also fix the syntax of the "interface-status-change" message.  Add
a null handler in the ThrottleService and Tethering classes (plus fix names).

Change-Id: I42cbed692024de32275cad234f42ff23ab7e9d8d
Signed-off-by: Mike J. Chen <mjchen@google.com>