Move mojom definitions of URL{Request,Response} into SDK

This moves the definition of the mojom URLRequest and URLResponse
objects into the SDK at //mojo/public/interfaces/network/. These are
used directly by the shell and have a special place in the system. This
patch does the minimal amount of import/include updating to get tests
passing but doesn't fully update everything - in particular it
doesn't update all the C++ code that #includes url_loader.mojom.h for
the URLRequest or URLResponse definitions to #include the more specific
mojom.h files, or the equivalent in other languages. I'll do that in a
follow-up to minimize churn.

R=viettrungluu@chromium.org

Review URL: https://codereview.chromium.org/1129063008

services/oknet/BUILD.gn

@@ -13,6 +13,7 @@ android_library("oknet") {
 
   deps = [
     "//base:base_java",
+    "//mojo/public/interfaces/network:network_java",
     "//mojo/public/java:bindings",
     "//mojo/public/java:system",
     "//mojo/services/network/public/interfaces:interfaces_java",

tests/services/network.sky

@@ -8,6 +8,7 @@ import "dart:sky.internals" as internals;
 import 'package:mojo/bindings.dart';
 import 'package:mojo/core.dart';
 import 'package:mojom/mojo/network_service.mojom.dart';
+import 'package:mojom/mojo/url_request.mojom.dart';
 import 'package:mojom/mojo/url_loader.mojom.dart';
 
 import 'package:sky/framework/embedder.dart';