Expose a way to create a test network cell in MobileCountries

In addition to exposing a static creator for a MobileCountries entity,
this adds equals/hashcode implementations to allow comparisons.

Bug: 393349646
Flag: EXEMPT will be used behind flag com.android.internal.telephony.flags.allow_multi_country_mcc
Change-Id: I6ae491450bd2f63429e9a7b2c7c83cfe6d0fdac0