audiopolicy: Add support for the MSD prototype

MSD is a Multi-Stream Decoder. It's an optional module
that allows mixing of encoded surround streams with
PCM streams.

This patch adds elementary support for setting up
a patch between the MSD module and actual output device,
and prioritizing the MSD module's device for output.

The patch is always set up, and is automatically
re-establishes itself if the actual output device changes.

Bug: 63901775
Test: MSD prototype
Change-Id: I9efa714455dae10b58bd5cc5cdf5557c0227b40c