set the country code when initializing the ot-daemon

The country code is lost after the ot-daemon is restarted. This
CL stores the country code to persistent settings and reads
out the previously stored country code to set the country code
when the ot-daemon is initialized.

Bug: b/326021472
Test: atest ThreadNetworkUnitTests

Change-Id: I6982a57ba4845f5a45f3366400c531398e82339e
9 files changed