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
Merged-In: I58eda22725dd4e67dc4b64207e38cf482032df10

(cherry picked from commit 605b2950)

Change-Id: I4d2be5d9037cfec03218b702d707f50808ee0694