AEStreamInfo: Handle sync headers with extension blocks

Fixes passthrough of Dolby TrueHD audio with Atmos.

Behavior based on FFmpeg patch "mlpdec: support major sync headers with
optional extension blocks" by Hendrik Leppkes.

Done as requested in: https://github.com/xbmc/FFmpeg/pull/2#issuecomment-57929385

@anssih: Thanks very much