Bluetooth: A2DP: Add AVRCP song position indicator

"sbc.avrcp_song_position" can be used to determine time synchronization
between AVRCP and A2DP. It is updated on every
AVRCP PlaybackPositionChanged. Also provide version for aptx.

Change-Id: I48cd49f0fee54131a738290e2a70a24d33ba1d22
Reviewed-on: https://code.wireshark.org/review/5290
Reviewed-by: Michal Labedzki <michal.labedzki@tieto.com>