MediaFocusControl: Deny audio focus if notification stream is 0

Previously audio focus would be granted regardless of notification
volume, meaning that if notifications are muted the media stream would
still be dimished for focus. By checking if STREAM_NOTIFICATION is 0 and
if a notification stream is requesting focus, this is no longer an issue.

Change-Id: Ie10a64e58ce77174a3bbdeaa936646d40c24404f
Signed-off-by: Evan Anderson <evan1124@gmail.com>