Ims: Fix for Phone App crash and NullPtrException in getCountryIso API

When back to back VOLTE calls are made, sometimes
next MO VOLTE call is failing due to a NullPointerException. The
issue is because of unexpected value in getting the current
country code. Added null check for detector.detectCountry(), if this
function call fails, then CountryIso will be picked up based on the Locale
CRs-Fixed: 478645, 490472

Change-Id: I9f35fe959fc072bf59a6aabc6c17dc4162426721