Retain progress for voicemail playback when rebinding voicemail card.

When rebinding the voicemail card, onPrepared gets called and resets the
voicemail playback to 0 because mPosition is not updated to the most
recent value. Update mPosition in this case to the position of the media
player and then call onPrepared.

Bug: 25080811
Change-Id: Ief3a021f4c11bac7b8c0a563765e14c897cb43b5