Add key to CarrierConfigManager

Add KEY_DEFAULT_VM_NUMBER_STRING and default value. Note that default
vm number no longer needs to be a string array, as gid is handled
within the new carrier_configs. Also remove references to old config.
Merged-In: I56e4e3713c27f449fd892db501b6097c8c89c3c6

Bug: 29873049
Change-Id: I56e4e3713c27f449fd892db501b6097c8c89c3c6