AudioService: simplified handler messages.

For historical reasons, messages to the AudioSystem
handler in AudioService have a complex identifier combining several information.
Messages for a particular stream contain the stream type as part of the message type.
This complexity is not needed as the stream messages also carry a reference to the
VolumeStreamState object which contains all necessary information, including the stream type.

Also grouped parameters of MSG_PERSIST_VOLUME message into a single bit field to make room for
further needs.

Change-Id: Ia27af626b3447541e8f9eaceba21389a824d88e2