Fix bug where 00:00 duration is shown for voicemails without a duration.

Instead of showing a 00:00 duration, nothing is displayed. When the
content is loaded, the duration field for the call log entry associated
with the voicemail is updated with the duration fetched from the
mediaplayer. The proper duration is then displayed in MM:SS format.

BUG=24175525

Change-Id: I1cafebae4fcbc749f573accfcf8833b598675f0b