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`.
(cherry picked from https://android-review.googlesource.com/q/commit:9ed86ce2ac19e2977393113578880279595a55fb)
Merged-In: Ife0c8df25df2fbe814cc6c8339c393ac0a897592
Change-Id: Ife0c8df25df2fbe814cc6c8339c393ac0a897592