Change voicemail number when roaming and IMS is not registered

When following conditions are fulfilled, device uses specific voicemail
number defined in
KEY_DEFAULT_VM_NUMBER_ROAMING_AND_IMS_UNREGISTERED_STRING.

1.Network is roaming.
2.IMS is in unregistered state.
3.KEY_DEFAULT_VM_NUMBER_ROAMING_AND_IMS_UNREGISTERED_STRING has any
  value(not empty).

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: I7a94619481061b35e8f55f2ab9f6e92706b16208