Set Thread Network country code from location country code

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