Voicemail ui indicates both time left and playback speed.

- Playback time indicator has changed from current position in
  voicemail to time remaining.
- This same text view is also used to briefly display current playback
  rate when playback rate buttons are pressed.
- Unit tests to show that this logic works.
- This cl introduces a new class whose job is to handle the transition
  between the different text states.
- Fixes a crash when we might release the player before the last
  update gets the duration from the released player.
- New test method to fetch a text view's content on the ui thread.

Bug: 5044075
Change-Id: Ie3cf6e58c1a0139edb78cf3564a8aec94512156f