Fix some voicemail playback bugs

* Don't fetch voicemail until playback is started
* Prevent multiple voicemail fetches for the same voicemail
* Fixed an issue where the seekbar would be stuck in the same
position even after seeking
* Fix for playback position getting reset after the voicemail
view is scrolled out of view and back in
* Respect the invariant that if mMediaPlayer is null, mPrepared
is always false. This fixes potential NPEs when resuming playback.

Bug: 22127956
Bug: 22333494

Change-Id: I5c419424950c0e21317cbd133ca6f7e1edd9fd31