[Thread] add country code overlay to enable/disable setting Thread country code

Some android devices call the API `setChannelMaxPowers()` to set the
Thread radio output power. Then there is no need for these devices
to set the Thread country code.

This commit adds an overlay configuration `config_thread_country_code_enabled`
to enable/disable setting the Thread country code.

Bug: b/402276117
Test: atest ThreadNetworkUnitTests:ThreadNetworkCountryCodeTest#initialize_countryCodeDisabled_defaultCountryCodeIsUsed
Change-Id: I215c64757d0277853f8d4625ace363a4d7bb974b
5 files changed