Better errors from unregisterNetworkCallback

This patch changes the validation of unregisterNetworkCallback in
ConnectivityManager so that the caller can better distinguish the case
of a callback that was never registered from the case of a callback that
has already been unregistered.

Bug: 62497809
Test: runtest frameworks-net passes
Change-Id: I58eda22725dd4e67dc4b64207e38cf482032df10
1 file changed