ConnectivityManager: a simpler CallbackHandler

This patch simplifies CallbackHandler in the following way:
  - CallbackHandler directly uses the static references to
    sNetworkCallback and sCallbackRefCount. This allows to remove
    instance fields in CallbackHandler.
  - CallbackHandler does not have a reference to ConnectivityManager
    anymore
  - CallbackHandler.getObject() is now generic in a type-safe way.

Test: ConnectivityServiceTest passes
Bug: 28537383
Bug: 32130437
Change-Id: I5004da5b91498e6ff7f8b05057a9e24b975bb56e