Check whether the WiFi country code is a valid Thread country code

The Thread country code should be a 2-Character alphabetical country
code. The `00` is an invalid country code for Thread country code,
but it is the default country code for WiFi country code. If setiting
the `00` as the Thread country code, the ThreadCountryCode throws
an exception.

This CL checks whether the country code is a valid Thread country
code before setting the WiFi country code as the Thread country code.

Bug: b/322404467
Test: atest ThreadNetworkUnitTests
Change-Id: Ic5fd69fc254ee25ad1e75e17096a5f5310a9140f