Register NetworkCallbacks to run in GnssNetworkConnectivityHandler thread

-Register network callbacks with ConnectivityManager to run in the
 mHandler thread instead of in the ConnectivityManager thread. Use of
 concurrent hashmap to track network attributes is not needed.
-Remove unnecessary methods.
-Mark public methods package private where applicable.

Bug: 123360691
Test: Verified with a real device that network updates are pushed to
      GNSS HAL implementation correctly. Verified SUPL connectivity.

Change-Id: I3c06d96bb2d0aa723d66369f3f11943d4a03fb19