Add logging and disconnect reason for null connections.

When a ConnectionService implementation returns a null connection, log this
and also set a unique disconnect reason to indicate in the telecom
dumpsys what happened.

Test: Manual
Bug: 70385625
Change-Id: Iff9846d434d400c4cf036e9ac46167cfb6f6b58c

telecomm/java/android/telecom/ConnectionService.java

@@ -1376,8 +1376,9 @@ public abstract class ConnectionService extends Service {
                 : onCreateOutgoingConnection(callManagerAccount, request);
         Log.d(this, "createConnection, connection: %s", connection);
         if (connection == null) {
+            Log.i(this, "createConnection, implementation returned null connection.");
             connection = Connection.createFailedConnection(
-                    new DisconnectCause(DisconnectCause.ERROR));
+                    new DisconnectCause(DisconnectCause.ERROR, "IMPL_RETURNED_NULL_CONNECTION"));
         }
 
         connection.setTelecomCallId(callId);