Add the system property threadnetwork.country_code

This CL follows the guide https://source.android.com/docs/core/architecture/configuration/sysprops-apis
to add the system property API `ThreadNetworkProperties.country_code()`
to read default Thread Network country code from the system property
`threadnetwork.country_code`. OEMs use this system property
to set the different Thread country codes for the product sold in
different countries.

The Resource Overlay is not used here for the follow reasons:
<1> The default country code needs to be set to different values for
    the same product selling to different countries. Resource Overlay
    doesn't work here as it would require a different RRO package
    for each country of of the same product.
<2> None of the other alternatives in https://source.android.com/docs/core/architecture/configuration
    can fulfill the requirement.
<3> Wi-Fi and UWB uses system properties for country code, so it will
    simplifies the build in factory if Thread is made consistent with
    them.

Bug: b/309357909
Test: Run `atest ThreadNetworkUnitTests`.
Change-Id: Ife0c8df25df2fbe814cc6c8339c393ac0a897592
diff --git a/srcs/android/sysprop/ThreadNetworkProperties.sysprop b/srcs/android/sysprop/ThreadNetworkProperties.sysprop
new file mode 100644
index 0000000..2b6ea33
--- /dev/null
+++ b/srcs/android/sysprop/ThreadNetworkProperties.sysprop
@@ -0,0 +1,24 @@
+# Copyright (C) 2024 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.
+
+owner: Platform
+module: "android.sysprop.ThreadNetworkProperties"
+
+prop {
+    api_name: "country_code"
+    type: String
+    prop_name: "threadnetwork.country_code"
+    scope: Public
+    access: Readonly
+}
diff --git a/srcs/api/PlatformProperties-current.txt b/srcs/api/PlatformProperties-current.txt
index 5a40147..70eef1a 100644
--- a/srcs/api/PlatformProperties-current.txt
+++ b/srcs/api/PlatformProperties-current.txt
@@ -475,6 +475,14 @@
   }
 }
 props {
+  module: "android.sysprop.ThreadNetworkProperties"
+  prop {
+    api_name: "country_code"
+    type: String
+    prop_name: "threadnetwork.country_code"
+  }
+}
+props {
   module: "android.sysprop.VndkProperties"
   prop {
     api_name: "product_vndk_version"