Audio V4: Do not send metadata of Output Track

Output track currently do not support audio attributes,
nevertheless the placeholders values were erroneously
propagated to the HAL.

Bug: 38184704
Bug: 69623109
Test: check that the HAL does not receive metadata about duplicated
      tracks.
Change-Id: I9a646faf4a43b1ebc09c03329827aee4a49674f9
Signed-off-by: Kevin Rocard <krocard@google.com>