Audio focus: start players in ducked state when focus is already lost

The volume ramp for ducking is only to be heard when a player is
  playing while focus is lost. When a player is starting when
  focus is already lost (and ducking is applicable), it should
  play at the same level as that reached after a duck. This is
  implemented using the VolumeShaper.Operation time offset
  (a.k.a. XOffset), set to 1, i.e. the end of the duck.

Test: see bug
Bug: 38353147
Change-Id: Iec36fbe002b183d6d70c258d9ab26b577dc1d2ce