[CS] Support "instant failure" from factories

Add a mechanism by which a factory can declare "instant failure" for
a request - which would result in it getting an OnUnavailable()
(even without a timeout).

Factories may only do this iff:
1. They know they are the only factory which may fulfill this
   request (common for transport-specific requests).
2. The know that the request can definitely not be
   fulfilled at any point in the future.

Bug: 31382922
Test: atest ConnectivityServiceTest
Change-Id: I9bce0f4d85fa8cad7f8a9998819f945b778c5ac5

core/java/android/net/ConnectivityManager.java

@@ -3138,9 +3138,9 @@ public class ConnectivityManager {
 
         /**
          * Called if no network is found in the timeout time specified in
-         * {@link #requestNetwork(NetworkRequest, NetworkCallback, int)} call. This callback is not
-         * called for the version of {@link #requestNetwork(NetworkRequest, NetworkCallback)}
-         * without timeout. When this callback is invoked the associated
+         * {@link #requestNetwork(NetworkRequest, NetworkCallback, int)} call or if the
+         * requested network request cannot be fulfilled (whether or not a timeout was
+         * specified). When this callback is invoked the associated
          * {@link NetworkRequest} will have already been removed and released, as if
          * {@link #unregisterNetworkCallback(NetworkCallback)} had been called.
          */