Background tasks can decide which executor to run with.

- Allow BackgroundTaskService to take an Executor as an optional argument,
  most commonly these will be one of the static fields defined on
  AsyncTask.
- Make voicemail prepare media on async background thread.
- Mark voicemails as read concurrently with other async tasks.
- Change background task service to accept an Executor as argument.
- Remove the initial progress and initial time from seek bar.

Bug: 5148472
Change-Id: I55815294fe671d3a42af0b2d6d5e29e6eaea271e