Make CALLBACK_PRECHECK actually work.

Currently, CALLBACK_PRECHECK is never fired, because we attempt
to send it just after a network has connected, before matching
requests to that network. So notifyNetworkCallbacks scans the
list of requests for the network, finds none, and does nothing.

Bug: 20038463
Change-Id: Ib0c6377c45990e358f926217d7c7defa74c1d59b