Hookup interface status to other clients

After unreverting the linkstate change patch, hook up notification handlers
that didn't exist when the first patch was created, like
EthernetDataTracker.java and Vpn.java.

For the observers that handle interfaceStatusChanged(), I made
interfaceLinkStatusChanged() call it so they both do the same thing.

Change-Id: I0077e5e5f48f3932ba98f5bf363243892f2de6cc
Signed-off-by: Mike J. Chen <mjchen@google.com>