AudioService: fix notifications with A2DP abs volume

When A2DP absolute volume is enabled, the music stream
software volume is maxed out because the headset applies
the volume.

We must also max out other streams playing over A2DP otherwise
they are attenuated twice and are barely audible.

Bug: 11316102.
Change-Id: I7e58c74cb683ad1d6f3b68abb0ef967c817fbf4f