Use a real MobileCountries instance in tests.

This change modifies tests to use MobileCountries.createForTest().

Test: atest FrameworksTelephonyTests
Bug: 393349646
Flag: com.android.internal.telephony.flags.allow_multi_country_mcc
Change-Id: I66fa38e4ee2be424f4de09e9d122e1a3d7d36a2c