Fix for wrong ringtone when using VoWiFi

If operator goes into flight mode and tries to make call via WiFi, then
property "gsm.operator.iso-country" is read as empty string and in
consequence mRegion is set as CEPT instead of specific country-area
name (e.g. instead of UNITED_KINGDOM).

We may check additional property that does not depend on network
operator: "gsm.sim.operator.iso-country" so that we could set mRegion
properly.

Bug: 29494265

Change-Id: Ia374f73159e005431fb0ad9bf8b5dbb3d9a3e294