Use framework API to match voicemail number.

We were tracking the voicemail number within the application. This has
two shortcomings: it would not be updated automatically when its value
changes and the matching against it was only based on it being
identical, without taking into account other possible formatting
differences.

Switch to use PhoneNumberUtils.isVoiceMailNumber() instead, which is
what the Phone app is also using.

Bug: 5435925
Bug: 5416495
Change-Id: Ie56363c578cf5f6102fd6880ffce14f608ddaf0b