A2DP encoding host: implement data position

* To allow the audio stack to get correct value from GetPresentationPosition
* Data position and read bytes values get reset everytime at StartRequest
* Remote delay value gets reset at end_session()
* CLOCK_MONOTONIC_RAW is used so the timespec value isn't affected
by NTP.

Tag: #floss
Bug: 192351589
Test: Build on Chrome OS. Execute
dbus-send --system  --type=method_call --print-reply \
  --dest=org.chromium.bluetooth /org/chromium/bluetooth/hci0/media \
  org.chromium.bluetooth.BluetoothMedia.GetPresentationPosition \
  string:<address>

Change-Id: Ife6fbc28f933cef636c90b66342163a631b4e91b