[rilmodem] Support for voice mailbox

Add support for voice mailbox (CPHS and MBDN). Enables speed dial to
voice mailbox from dialer and importing voice mailbox number to Settings
application.

Signed-off-by: Petri Takalokastari <petri.takalokastari@oss.tieto.com>
Merge pull request #85 from ptakalok/master

[rilmodem] Support for voice mailbox