Zero out return values in audio binder calls

More specifically when handling GET_OUTPUT_FOR_ATTR in IAudioPolicyService.
This prevents leaking a uninitialized `output` across binder if
getOutputForAttr were to encounter errors.

Bug: 23756261
Change-Id: Ibff8a1249a4e8a3c89a33a540dda428b10d6ca82
(cherry picked from commit a946d844)
Ticket: CYNGNOS-1189