audioflinger: fix duplicating thread output buffers clear

Also clear mix buffer in DuplicatingThread::threadLoop_mix()
instead of just sink buffer when output threads are not
ready.

Bug: 18262127.
Change-Id: I4c22bbeaccdcc8d7c10c5882dde3439eb73d6ede
(cherry picked from commit 02b5708776ba2a9b4ff8c09008483aef7dbe38c7)