Check that default voice ID is valid before using

When migrating IMS settings, we use the default voice subId to construct
a URI for the subinfo db. The URI must contain a valid subId, so we
first check that the subId is valid.

Fixes: 130246978
Test: manual and SubscriptionControllerTest#testSkipMigrateImsSettings
Change-Id: Ib7b12bae9ea2cd60d16b488fd9dbb37e0643dda1