When dialing voicemail, check voicemail number of correct default SIM.

Before dialing voicemail when a default SIM is set, Dialer checks first if it
has a valid voicemail number. However, the previous code was checking the
voicemail number of the first SIM as opposed to the default SIM. This
change corrects the logic.

Bug: 18938231
Change-Id: Ia83aa44bc773e99aabbe7474439d22eb1c4e6fd9