Add CarrierConfig of voicemail number for roaming and IMS unregistered

This CarrierConfig is used to configure voicemail number while the
device is both roaming and not registered for IMS.

Bug: 118729539
Test: manual - Checked that voicemail number is changed according to
KEY_DEFAULT_VM_NUMBER_ROAMING_AND_IMS_UNREGISTERED_STRING, when IMS is
in unregistered state, and network is roaming, and
KEY_DEFAULT_VM_NUMBER_ROAMING_AND_IMS_UNREGISTERED_STRING is not empty.
Change-Id: I16bfe697c6ffa01be9f00695e8c4524686c767d7