audio: Handle sonification for last stream in stopOutput

-Low visibility stream is not muted during voice call when
played back to back.
-In startOutput, handleInCallSonification is called only for
first time i.e. mRefCount is 1. While in stopOutput, it is
called without any check on refCount.
So the second notification stream gets played when stopOutput
of first stream unmutes the muted stream.
-Handle sonification while in call only for last stream in
stopOutput i.e. mRefCount is 0.

CRs-Fixed: 645543
Change-Id: I73727c3cc39e7ceecfbc961c3accb109cd013d96