Hopefully fixed inconsistencies with resuming after events.

The events in question were:
  - Headphone jack plug/replug
  - Playback interrupted by a phone call
  - Audio Focus being lost/gained (still a little fuzzy on this one)

The "PauseCause" is now kept track by an enum of the same name.
Each of the three cases should only be the one to resume playing if it was the one to pause it.