Simplified data network controller callback

Aligned with other data modules, now DataNetworkController
also uses DataCallback. ServiceStateTracker registers
data network existing state changed event from the beginning
of the life cycle.

Fix: 211913235
Test: manual + atest ServiceStateTrackerTest DataNetworkControllerTest
Merged-In: I779b389f15e46bd3f27bcee2adf0bcb48654a3ca
Change-Id: I779b389f15e46bd3f27bcee2adf0bcb48654a3ca