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.
This causes second notification stream to be played when
stopOutput of first stream unmutes the muted stream.
-Handle sonification while in call only for last stream in
stopOutput i.e. only if mRefCount is 1.

CRs-Fixed: 663101
Change-Id: If6cbf24aa814a115c98358883794a7c92aec0c86