audio: no notification sound when adjusting volume bar twice.

Notification sound won't take effect when the previous track hasn't
finished.
Need explicitly stop the previous ongoing track to alow the current one.

Change-Id: Ib344a8b14ecc47a5c8fdd9c11ae48a3541ea9a55
CRs-Fixed: 652236
Merge "audio: no notification sound when adjusting volume bar twice." into du44