Add the system property threadnetwork.country_code

This CL follows the guide
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
    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

Bug: b/309357909
Test: Run `atest ThreadNetworkUnitTests`.
Change-Id: Ife0c8df25df2fbe814cc6c8339c393ac0a897592
2 files changed
tree: bf88fba09d6f0385b840a1bc208a0ba6553c5cfa
  1. srcs/