topshim: a2dp - Expose GetPresentationPosition dBus method

- Make a2dp intf functions const.

Bug: 189497374
Tag: #floss
Test: Connect headset and start A2dp streaming.
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: I890329dfe2d5337252a31c7277fc249cb3e08817