Call.Decorator network pinning on Android

[yschimke]

Cleanly pinning network on Android - #8286

val pinning = AndroidNetworkPinning()
val client = OkHttpClient.Builder().addCallDecorator(pinning).build()

// clean up lost networks
connectivityManager.registerNetworkCallback(NetworkRequest.Builder().build(), pinning.networkCallback)

// Substitute in some preferred network here
val activeNetwork = connectivityManager.activeNetwork

val request = Request.Builder()
  .url(server.url("/"))
  .tag<Network>(activeNetwork)
  .build()

To land after #8800