setPreferedDevice considered ahead of Dynamic Rules

Audio policy is updated to allow an applications expressed preference
for a specific audio device to override dynamic policy rules, just as it
already overrides the rest of the policy engine's logic.
This is important in allowing 1st party automotive applications to
explicitly targets specific audio buses.

Test: build and run on Mojave
Change-Id: I482b74cba40f6d6150f69856769a998158a54a94