'availability_time_offset' is not an attribute in MPD specification,
but used to calculate availability_start_time to adjust segment
availability time. A similar effect can be achieved with standard
blessed suggested_presentation_delay flag.
If suggested_presentation_delay is not set, player is expected to
choose a suitable value.
Change-Id: I1d540bd4347b4f04b6cc5ec0778e1a5392009b09