audio policy: fix regression with duplicated output handle

Commit fe231127 made that IO Handle for duplicated outputs
were not allocated properly causing volume commands to be sent
with an invalid IO handle to audio flinger.
We then hit another problem with a legacy behavior which broadcasts volume
commands to all output threads if received with IOHandle 0.

Bug: 70332110
Test: manual test.
Change-Id: Ib133abab1575e3c90ba235f0a52d5ab845514719