audio policy: suppport for dynamic source

Completed support for dynamic sources:
- remote submix input device is dynamically connected/disconnected
when corresponding AudioTrack starts/stops.

Also do not create a duplicated output for mixes used by dynamic policies.

Bug: 16006090.
Change-Id: Ib5b5a2159fcac9d9f410a5a2f1c64d7ebad79f72