Raise the transmission priority for a2dp media channel to high

Raise the transmission priority for a2dp media channel to high
so that when a2dp streaming runs simultaneously with some other
b/w consuming operation such that file transfer over BT, a2dp
media link is given more priority over other channels so that
host does not delay sending the media packets.

CRs-Fixed: 628669

Change-Id: I42e996824d786a3ef8ff928a2c7cd31552cb8834