When we leave the voicemail activity, stop playback.

- On pause is propagated through to the VoicemailPlaybackPresenter.
- We use this to stop playback.
- Added a test used when working with this, but suppressed it because
  I can't find a simple-enough way of checking that the audio has
  stopped.
- Adds the ability to test the CallDetailActivity with a real voicemail.
- Fixes to the FakeAsyncTaskExecutor that Flavio and I have already
  discussed - fixes that make the onPreExecute() method work correctly
  (i.e. get invoked immediately on the thread that calls submit() on the
  executor, rather than on the same thread that calls run... from the
  test, which is a poor simulation and also illegal).

Bug: 5206675
Change-Id: I3f96c438dc6bbc1413a40b7bb2306ceb413f14a8