This CL selects the Thread country code from location country code.
If the location country code is not avaliable, the country code `WW`
will be selected as the default Thread country code.
This CL also adds Shell commands for developers to override the Thread
country code for testing.
Bug: b/309357909
Test: Run `atest ThreadNetworkUnitTests`.
Change-Id: Id87c293005f0e75922a72854b40c41837b74397f