audio flinger: report actual sink device for MSD playback threads

When a playback thread is routed to an MSD module, make sure that the
actual sink device selected by the software patch at the output of the
MSD module is reported to AudioTrack clients.

Bug: 139894721
Test: atest AudioTrackTest
Test: atest RoutingTest
Change-Id: Ie52ebe198e9ebdfe5f2a38dcd44c551c705a7931
Merged-In: Ie52ebe198e9ebdfe5f2a38dcd44c551c705a7931