Audio V4: Update .hal doc to removal of audioSource

As multiple tracks can be connected to the same stream,
the audio source list can be found in metadata.

Bug: 38184704
Test: none
Change-Id: I63e75fd4049fbe469b78cacf0adf44e9597cb422
Merged-In: I63e75fd4049fbe469b78cacf0adf44e9597cb422
Cherry-picked from master
Signed-off-by: Kevin Rocard <krocard@google.com>