Prepare to delete useless loop in handleReleaseNetworkRequest.

As explained in the TODO, the loop serves no purpose since only
one network can be satisfying a given request at a time.

Instead of looping, look up the nai in the mNetworkForRequestId
array that exists for this purpose.

Keep the loop around with an Slog.wtf statement on it so we can
see if we ever hit it, and add a TODO to delete it if we don't.

Bug: 23113288
Change-Id: I173de4bd45c5a4169b7a062a981f2ecccaa44143