Expose user home country code as public method in Mediator Interface.

This will be used in a subsequent change to show the user
the default/predetected country code in the assisted dialing
settings.

Bug: 68775522
Test: unit tests
PiperOrigin-RevId: 177168946
Change-Id: Ic738a6ddde8766962a725668f6db5056ab0bcafd