Fix ConnectivityManagerApi23Test failures and remove duplication.

1. All ConnectivityManagerApi23Test were failed due to
   WifiManager#setWifiEnabled doesn't allow to use since
   Android Q. So we need to use shell command to enable/disable
   Wi-Fi instead.
2. Some methods are duplicated between
   ConnectivityManagerApi23Test and ConnectivityManagerTest, but
   they are not identical. So put these methods into
   ConnectivityUtils to clean up duplications and prevent fork
   happened again.

Bug: 133334943
Bug: 133209319
Test: Run the below tests on Crosshatch, Sailfish, Bonito.
      atest CtsNetApi23TestCases
      atest CtsNetTestCases

Change-Id: Ic37111cb12a46f5c36c2be887250c5d762216f6e
diff --git a/tests/cts/net/util/Android.bp b/tests/cts/net/util/Android.bp
new file mode 100644
index 0000000..1f94613
--- /dev/null
+++ b/tests/cts/net/util/Android.bp
@@ -0,0 +1,25 @@
+//
+// Copyright (C) 2019 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+// Common utilities for cts net tests.
+java_library {
+    name: "cts-net-utils",
+    srcs: ["java/**/*.java", "java/**/*.kt"],
+    static_libs: [
+        "compatibility-device-util-axt",
+        "junit",
+    ],
+}
\ No newline at end of file