Fixing failing NitzStateMachineImpl tests

Changes in this commit:
* Changes the NitzStateMachineImpl to clear the mobileCountries variable when the network state is unavailable.
* Changes the NitzStateMachineImpl to reset mCountryIsoCode and mMobileCountries when a country is reset.
* Update tests that were only implemented in the main branch (i.e. allow_multi_country_mcc = false)

Bug: 393349646
Test: atest FrameworksTelephonyTests
Flag: com.android.internal.telephony.flags.allow_multi_country_mcc

Change-Id: I0b1f68062970e7c180f2348246118a692fa31482