Zero out return values in media binder calls

More specifically when handling:
* GET_STREAM_VOLUME in IAudioPolicyService, and
* GET_CURRENT_POSITION and GET_DURATION in IMediaPlayer

This prevents leaking uninitialized values across binder in error cases.

Bug: 23756261
Change-Id: I0ffd900ab12b685b0611259ade4a3efb1ec5defe