Clean up APN notifications.

Add APNType info to notifications so you can tell what's happening.  Now, even if a new APN
shares a connection with an already-connected-to- apn type, the new type will get all
the connecting and connected messages on connect and disconnecting/disconnected on disconnect
even though the shared connection remains connected.

Cleaning out the hacks MobileDataStateTracker needed to deal with the old situation.
bug:2226092

Change-Id: Iddd7421d6b91cda7c8405f9c3d5404ac04ef8e42