Resume voicemail playback at previous position on resume.

Make sure that the position is saved when navigating away from the
voicemail playback so that it can be restored when resuming. Also:
- Disable seekbar when not loaded
- Gray out seekbar handle when seek is disabled

Bug: 23566924

Change-Id: Ic9d84425d7a3cde9d212bd758eb518577161d7ec