Support to load default voicemail number from the configuration.

If there is no preload VM number in the sim card, carriers such as
Verizon require to load a default vm number from the configurantion.
Define config_default_vm_number for this purpose. And there are two
optional formats for this configuration, example as below:
(1)<item>voicemail number</item>
(2)<item>voicemail number;gid</item>

Bug: 17209959
Change-Id: Ia4e78ad7265e73bb4e4a823dbaeb606935d39f46